Secretary of State Hillary Clinton has met with South Korea's president and other top officials, as efforts get underway to rally global support for measures against North Korea. Western investigators accuse the North of a deadly naval attack.
Secretary of State Clinton took a positive approach Wednesday toward what many view as an uphill battle in convincing China to help punish North Korea. "I believe that the Chinese understand the seriousness of this issue and are willing to listen to the concerns expressed by both South Korea and the United States," she said.

A multinational investigative team concluded last week a torpedo fired from a North Korean submarine was responsible for sinking a South Korean patrol ship in March, killing 46 sailors. Clinton expressed what she called "greatest admiration" for South Korea's restraint in avoiding an emotional response. She says the thorough and scientific probe it conducted should be helpful in persuading China.
"We hope that China will take us up on our offer to really understand the details of what happened and the objectivity of the investigation that led to the conclusions," Mrs. Clinton stated.

China is a permanent member of the United Nations Security Council, where South Korea intends to call for new international sanctions against Pyongyang. It has been a historical lifeline of food and fuel to impoverished North Korea, but -- on a more practical level -- is reluctant to do anything that might destabilize the country and create a security threat right next door.

In a joint news conference with Clinton, South Korean Foreign Minister Yu Myung-hwan said he too believes the investigation report will bring even hesitant nations around.

He says it may take time in the case of China and Russia, but that they cannot deny the facts forever.

Beyond punishing the North for unacceptable behavior, Secretary Clinton called upon the international community to get North Korea to change course. "There's a different path for North Korea. And, we believe it's in everyone's interest, including China, to make a persuasive case for North Korea to change direction," she said. "They need to look internally toward what they can do to improve the standing of their own people and provide for a different future."

Chinese Premier Wen Jiabao is scheduled to arrive in Seoul on Friday.

North Korea has begun to freeze ties with the South, which already halted most trade with Pyongyang in retaliation for the sinking of a South Korean warship. The North has denied responsibility for the attack on the vessel and is accusing the South of launching a "smear campaign" against it.
Pyongyang has expelled eight South Korean government officials from a joint factory park in the North. And, it is threatening to block what little cross-border traffic exists.

The Unification Ministry in Seoul says hundreds of South Korean managers and other workers from the South were allowed to enter the industrial complex in the west coast Kaesong border city, Wednesday.

But ministry spokesman Chun Hae Sung tells reporters North Korea quickly acted on other aspects of its threat to cut all communications ties with the South.

He says Pyongyang Wednesday halted contact between the Red Cross delegations in the truce village, Panmunjom, and the North Korean Navy contacted the South to inform it that all marine communications between the two Korea's are now cut.

Relations between the two Koreas have deteriorated steadily since the Cheonan, a South Korean naval vessel in the Yellow Sea, exploded a month ago, killing 46 crew members. An international investigation concluded last week that the coastal patrol warship was hit by a North Korean torpedo.

South Korea's defense ministry tells VOA News that plans to send tens of thousands of leaflets northward by ballon have been delayed because of wind conditions, but they could go aloft as early as Thursday. Officials say the leaflets are intended to inform North Koreans about the sinking of the South Korean naval vessel. The North views Southern pamphleteering as hostile propaganda.

South Korea's military is using loudspeakers along the border, silenced for six years, and re-instituting FM broadcasting to the North.

North Korea's state television newscaster announced such propaganda will not be tolerated.

The North Korean newscaster says it will open fire on the South Korean loudspeakers and destroy them.

Pyongyang says a resumption of the propaganda campaign will also compel it to totally shut down the Kaesong industrial complex, where more than 100 South Korean firms employ about 42,000 North Korean workers.

The two countries have no diplomatic relations and technically remain at war following a 1953 truce which ended the three-year Korean War.

The United States, which has 28,000 troops in South Korea, has hurriedly announced plans for several joint military exercises in the coming month. In the past, Pyongyang has strongly condemned U.S.-South Korean drills, claiming they are preparations for an invasion of the North.

North Korea has angrily rejected a multinational investigative team's findings that it was responsible for the deadly sinking of a South Korean naval ship. The investigators says it is beyond doubt a North Korean submarine fired a torpedo and split the ship in half.

North Korea's National Defense Commission issued a statement Thursday calling the international team's findings "sheer fabrication," and a "conspiratorial farce." It warns Pyongyang will meet any retaliation with "tough measures, including an all-out war." The North is also vowing to send a team to South Korea to inspect the evidence for itself.

Just a short while earlier, the international team made public its conclusion that North Korea was involved in the sinking of the Cheonan, a South Korean warship. It marked the first time South Korea has explicitly and publicly blamed North Korea for the sinking.

Yoon Duk-yong is the South Korean co-director of the team. He says it is clear the sunk by a torpedo made in North Korea. He says the evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine. He says there is no other plausible explanation.

Yoon says his team can also confirm a submarine team left a North Korean naval base two or three days before the attack, and returned home two or three days after the ship was sunk.

The team went into deep forensic detail. South Korean military intelligence analyst Hwang Won-dong pointed to what he says are recovered pieces of a torpedo with Korean language markings. He says he knows of no other nation besides North Korea that would mark their torpedoes using the Korean alphabet.

Lee Geun-duk, an explosives analyst, pointed to residue found both on the recovered torpedo parts and on the damaged hull of the Cheonan. He says the team determined the substance found on the torpedo propeller and on the Cheonan are identical.

The Cheonan investigative team brought in analysts from the United States, Australia, the United Kingdom and Sweden. U.S. Navy Rear Admiral Thomas Eccles, who directed the American investigative team, says none of the nations involved in the probe dispute its conclusions.

"The international team, in close cooperation with the Republic of Korea joint investigative group, worked both in a collaborative way - very closely together - and also employing our separate tools and methods... and in all of those, we found agreement both within the Republic of Korea and all of the international team," said Eccles.

Forty-six South Korean sailors were killed in the sinking of the Cheonan. A South Korean military reprisal is seen as almost impossible, because of the chance that action could escalate into a devastating war. South Korea says it will call for coordinated international action via the United Nations Security Council and the United Nations Commission that monitors an armistice between the two Korea's.

World leaders are heading for Washington to discuss what Barack Obama has described as "the most immediate and extreme threat to global security" - the risk that terrorists could acquire a nuclear bomb. But how likely is this scenario?

A former investigator with the CIA and the US department of energy, Rolf Mowatt-Larssen, says there are three headlines that keep him awake at night:

• Pakistani 'loose nukes' in the hands of terrorists

• North Korea supplies terrorists with nuclear bombs

• Al-Qaeda launches nuclear attack

The good news is that he thinks "the odds are stacked against" terrorists acquiring a nuclear bomb.

But the low probability, he argues, has to be weighed against the awfulness of the consequences.

In today's unpredictable world, he writes in an article for the Bulletin of the Atomic Scientists, "a probability-based approach to managing risk" makes less sense than one "focused on mitigating threats in descending order of their possible consequences".

It's an argument that Barack Obama was making long before his election.

"Instead of taking aggressive steps to secure the world's most dangerous technology, [the US has] spent almost $1 trillion to occupy a country in the heart of the Middle East that no longer had any weapons of mass destruction," he said in a speech at Purdue University, Indiana, in July 2008.

Three months later, a commission set up by the US Congress warned that without decisive action it was "more likely than not" that a terrorist attack involving WMD would occur by the end of 2013.

Pakistan

In Rolf Mowatt-Larssen's view, there is "a greater possibility of a nuclear meltdown in Pakistan than anywhere else in the world".

The region has more violent extremists than any other, the country is unstable, and its arsenal of nuclear weapons is expanding.

Once a new plutonium reactor comes on line in the near future "smaller, more lethal plutonium bombs will be produced in greater numbers", he says.

The possibility of a Taliban takeover is, he admits, a "worst-case scenario".

But the Taliban and al-Qaeda are not the only shadows on the Pakistani landscape. There is also the Lashkar-e-Taiba militant group, which is accused of carrying out the Mumbai attack in November 2008, and like the Pakistani officer corps, recruits mostly in the Punjab.

"As one senior Pakistani general once told me," wrote Bruce Riedel of the Brookings Institution last week, "the relationship between the army and the Lashkar-e-Taiba is a family affair".

He went on: "Pakistan has taken serious measures to protect the crown jewels of its national security, but it lives in a perilous time. If there is a nightmare nuclear security scenario in Pakistan today it is probably an inside-the-family-job that ends up in a nuclear armageddon in India."

The point is echoed by Ian Kearns of the British American Security Information Council (Basic), who writes of the danger that states could use terrorist groups to attack adversaries "by proxy", engineering nuclear security breakdowns to facilitate terrorist access to weapons or materials.

BBC correspondents say there is every indication that the Pakistani military is in total control of the country's nuclear facilities.

North Korea

The reason North Korea keeps Rolf Mowatt-Larssen awake at night is connected with the mysterious site at al-Kibar in Syria, destroyed by Israeli missiles in 2007.

It's his view that North Korea was helping Syria build a reactor there and that the outside world only found out because of a "windfall of intelligence".

"Taking into account the sobering reality that Kim Jong-il came close to providing Syria with the building blocks for nuclear weapons... how confident can the international community be that there is not a long-running 'AQ Kim' network in North Korea that is analogous to the AQ Khan rogue state nuclear supplier network in Pakistan?" he asks.

The episode showed, in his view, that it is hard enough for the intelligence community to spot state-related clandestine nuclear activity, let alone clandestine nuclear trafficking of non-state actors, which would have a much smaller footprint.

North Korea's "erratic and irresponsible behaviour" makes it a leading potential source for terrorists seeking to acquire nuclear-related technologies and materials, he says.

Al-Qaeda

Though he now works in academia, Rolf Mowatt-Larssen led US efforts to determine whether al-Qaeda possessed a nuclear bomb, in the wake of 9/11.

He doesn't believe it does. But "the group's long-held intent and persistent efforts to acquire nuclear and biological weapons represent a unique means of potentially fulfilling their wildest hopes and aspirations," he writes.

Al-Qaeda's experience on the nuclear black market has taught its planners that its best chance lies in constructing an "improvised nuclear device (IND)," he says.

For this they would need either a quantity of plutonium or 25kg-50kg of highly enriched uranium (HEU), the size of one or two grapefruits.

HEU is held in hundreds of buildings in dozens of countries. "Security measures for many of these stocks are excellent, but security for others is appalling," according to a report published in 2008 by the Nuclear Threat Initiative.

The IAEA registered 15 confirmed cases of unauthorised possession of plutonium or HEU between 1993 and 2008, a few of which involved kilogram-sized quantities. In most cases the quantity was far lower but in some cases the sellers indicated there was more. (If there was, it hasn't been traced.)

There is no global inventory of either material, so no-one can be sure how much has gone missing over the years.

Neither are there agreed international standards for security and accounting of these materials. UN Security Council Resolution 1540 merely calls for "appropriate and effective" measures, without defining this in detail.

"It is a stark and worrying fact, therefore, that nuclear materials and weapons around the world are not as secure as they should be," writes Ian Kearns, in his Basic report.

The main goal of the Washington summit is to make progress on this issue.