The bodies representing most European airlines and airports have questioned the need for the unprecedented curbs, which affect millions of travellers.

Airlines that have carried out test flights say planes showed no obvious damage after flying through the ash.

EU Transport Commissioner Siim Kallas said he hoped 50% of Europe's airspace would be risk-free on Monday.

He said the current situation was "not sustainable" and European authorities were working to find a solution that did not compromise safety.

Mr Kallas also said EU transport ministers would hold a video teleconference on Monday to assess the situation. About 17 European countries have closed their airspace.

The flight bans came amid fears that the ash - a mixture of glass, sand and rock particles - can seriously damage aircraft engines. Airlines are estimated to be losing some £130m ($200m) a day.

ACI Europe - which represents major airports - and the Association of European Airlines issued a joint statement urging officials to reconsider the restrictions.

'Dangerous levels of ash'

"The eruption of the Icelandic volcano is not an unprecedented event and the procedures applied in other parts of the world for volcanic eruptions do not appear to require the kind of restrictions that are presently being imposed in Europe," the statement said.

Earlier, several airlines also questioned the curbs.

Peter Hartman, chief executive of Dutch carrier KLM, said there had been "nothing unusual" about a test flight carried out by the airline through the plume, and he hoped to "get permission as soon as possible to partially restart our operations".

Steven Verhagen, vice-president of the Dutch Airline Pilots Association, told the Associated Press news agency: "In our opinion there is absolutely no reason to worry about resuming flights."

Germany's two biggest airlines, Lufthansa and Air Berlin, also said they had carried out test flights without apparent damage, as did Air France.

Air Berlin spokeswoman Diana Daedelow told the BBC: "It is astonishing that these findings... have seemingly been ignored in the decision-making process of the aviation safety authorities."

A British Airways Boeing 747 completed a test flight through the no-fly zone at 30,000ft (9.1km) from Heathrow to Cardiff on Sunday.

While it encountered no problems, no loss of engine performance and no damage to windows, engineers in Cardiff were due to make a more detailed assessment of its engine overnight.

Earlier on Sunday, a UK Met Office plane went through the cloud and encountered dangerous levels of ash, showing that the issue is not whether the cloud is real and dangerous but whether its extent can be accurately mapped, BBC business editor Robert Peston writes.

One possible solution is to put observation planes in the sky, to give a more detailed picture of the location of ash concentrations, and the UK government is therefore trying to obtain more observation planes, from the military in particular, he adds.

Our business editor understands that BA fears it may not be allowed to fly normal services until Thursday at the earliest.

Worsening disruption

UK Transport Secretary Lord Adonis, said "urgent discussions" were taking place between European and international agencies to ease the chaos.

"We want to be able to resume flights as soon as possible, but safety remains my paramount concern," he said.

Weather experts say wind patterns mean the cloud is not likely to move far until later in the week.

Brian Flynn, head of operations at Eurocontrol - which co-ordinates air traffic control in 38 nations - dismissed suggestions the authorities were being over-cautious.

"With the overriding objective of protecting the travelling public, these exceptional measures have to be taken," he said.

Eurocontrol said there were only 5,000 flights in European airspace on Sunday, against 24,000 normally. There was a similar figure on Saturday. All but 55 of 337 scheduled flights by US carriers to and from Europe were also cancelled.

China has been sifting rubble for survivors of the deadly quake which hit the remote Qinghai region as thousands spent a freezing night in the open.

Rescuers used bare hands and picks to search debris as night fell, with little heavy lifting equipment in the mountainous, mainly Tibetan area.

Officials say 589 people died and 10,000 were injured when the quake struck Yushu county early on Wednesday.

But the death toll is expected to rise and further aftershocks are feared.

Relief flights carrying medical workers and supplies have been landing in Yushu airport but the road to the town of 70,000 people has been blocked by a landslide, the Associated Press news agency reports from Qinghai's provincial capital, Xining.

The BBC's Chris Hogg, in Qinghai and travelling by road to the disaster area, passed long columns of military vehicles carrying diggers and other heavy lifting equipment as dawn was breaking on Thursday.

The columns, which included ambulances, were about eight or nine hours from the disaster zone, he said.

In the township of Jiegu, 85% of buildings were destroyed, officials say, and state TV has been showing street after street reduced to rubble.

Several schools collapsed and at least 56 students are known to have died, 22 of them in a school in Yushu.

Chinese President Hu Jintao has called for an all-out effort to save as many people as possible and some 5,000 rescuers, including 700 soldiers, have been sent to the disaster area.

The civil affairs ministry said it was to send 5,000 tents, 50,000 coats and 50,000 quilts as local officials in Yushu reported a lack of tents, medicines and medical equipment.

China has received messages of sympathy and offers of aid from foreign states including Japan, Russia and France.

'So much dust'

About 900 people have been pulled alive from under the rubble since the quake struck at 0749 on Wednesday (2349 GMT Tuesday), at the shallow depth of 10km (six miles), Chinese media say.

But Wu Yong, a local army commander, said the death toll could rise "as lots of houses collapsed".

Rescue operations were being hampered by the fact that the magnitude 6.9 quake disrupted telecommunications, knocked out electricity and triggered landslides.

In Xining, some 860km (530 miles) from the quake zone, soldiers, fire-fighters and rescue workers with sniffer dogs thronged the airport, which closed to civilian flights for several hours to make way for relief planes.

Efforts are being slowed down by the lack of jet fuel stored at Yushu airport. Relief planes are having to carry extra fuel, limiting their space for supplies.

As local officials struggled to find accommodation for the thousands of people left homeless, weather forecasters were predicting wind and sleet in the coming days, putting victims at risk of exposure.

Luo Song, a monk from a monastery in Yushu county, said his sister who worked at an orphanage there had told him three children were sent to a hospital but the facilities lacked equipment.

"She said the hospitals are facing a lot of difficulty right now because there are no doctors, they have only bandages, they can't give injections, they can't put people on intravenous drips," he told AP by phone from the southern Chinese city of Shenzhen.

In Jiegu, hotel manager Ren Yu said that nearly all of the local mud and wood houses had collapsed.

"There was so much dust in the air, we couldn't see anything," he said.

"There was a lot of panic. People were crying on the streets. Some of our staff, who were reunited with their parents, were also in tears."

Harrowing photographs have emerged of emergency workers removing dust-covered dead infants from rubble.

The high-altitude region is prone to earthquakes but, according to the US Geological Survey, this was the strongest tremor within 100km of the area since 1976.

In 2008, a huge quake struck neighbouring Sichuan province, about 800km from Yushu. That left 87,000 people dead or missing and five million homeless.