A plume of ash measuring up to seven kilometers (more than four miles) high had been detected at the Eyjafjoell volcano, said a statement late Thursday from the Icelandic Met Office and Institute of Earth Science.

"The eruption has changed back to an explosive eruption, lava has stopped flowing and most of the magma gets scattered due to explosions in the crater," said the statement in English.

"The ash plume rises high above the crater (4-7 km) and considerable ash fall can be expected in wind direction. No signs of the eruption ending soon."

The move prompted authorities to consider Thursday re-routing transatlantic flights and Ireland to shut airports for a third time this week.

"Ash production did increase last night and the ash plume is going higher now than the last couple of days," Agust Gunnar Gylfason, who monitors the eruption's progress at Iceland's Civil Protection Department, told AFP.

European airspace and airports across the continent were open Thursday, but intergovernmental air traffic control agency Eurocontrol said the ash cloud could mean transatlantic flights might need to be re-routed.

"Significant re-routing of westbound transatlantic flights to avoid the higher contaminated area is currently being discussed between the air navigation service providers concerned and Eurocontrol," the Brussels-based body added.

The area where concentrations of ash could still pose a risk to aircraft engines was lying to the north and west of Ireland, it said.

While the airspace above Ireland, Northern Ireland and Scotland was open on Thursday after having been shut for the previous two days, Irish authorities announced later in the day the new ash had forced them to shut airports again.

The Irish Aviation Authority (IAA) warned a "massive ash cloud" 1,000 miles (1,600 kilometres) long and 700 miles wide was hovering over the Atlantic and drifting into the country's airspace.

"While the northerly winds are keeping the bulk of the cloud out in the Atlantic, the increased size of the cloud is encroaching on Irish airspace along the west coast of Ireland," said the regulator in a statement.

Six airports would be closed, starting at midnight (2300 GMT) Thursday and continuing until 1:00 pm (1200 GMT) Friday. Among those shutting down were Shannon and Donegal, said the IAA.

The fresh disruption came after Europe's skies were closed for up to a week last month by the eruption of Eyjafjoell. It was the biggest aerial shutdown in Europe since World War II, with more than 100,000 flights cancelled and eight million passengers affected.

Gylfason said ash plume "had reached 30,000 feet (9,000 metres) for some time last night, and again this morning, due to an increase in explosive activity, but otherwise it's been around 18,000 and 20,000 feet."

At the strongest period of the eruption, Eyjafjoell sent a plume around 30,000 feet into the air, but scientists have stressed that the height of the plume does not necessarily reflect a particular quantity of ash.

On Tuesday, the plume contained about only 10 percent of the ash it held at the beginning of the eruption.

British Airways said Thursday that its passenger numbers fell by almost one quarter last month as a result of the flight bans prompted by the volcanic ash cloud.

Its total number of passengers slumped 24.5 percent to 2.081 million people in April, compared with the same month of 2009, it said in a statement.