A huge cloud of ash from an Icelandic volcano covered northern Europe on Thursday, grounding thousands of flights as countries imposed the biggest airspace closure since the September 11 attacks in 2001.

Fallout from the Eyjafjallajokull volcano in southeast Iceland could take several days to clear, experts warned, after the eruption had already melted the 250 metre (820 feet) thick glacier around it, causing severe floods.

With thousands stranded in airports around the world, Eurocontrol, the European air traffic control group, said planes could stay grounded for at least 48 hours.

It estimated that overall 5,000 to 6,000 flights were likely to have been cancelled on Thursday as grey ash from the second major eruption in Iceland in less than a month blew across the north Atlantic, closing major airports more than 1,300 miles (2,100 kilometres) away.

Eurocontrol predicted that at least half of the daily 600 flights between Europe and North America would be cancelled on Friday.

Belgium, Britain, Denmark, the Netherlands, Norway and Sweden shut down their airspace because the ash was a threat to jet engines and visibility. There was also major disruption in Finland, France, Germany and Spain.

The cloud spread over northern Poland late Thursday forcing aviation authorities to close airspace over the north of the country.

The memorial and funeral services for President Lech Kaczynski, who was killed in a plane crash, is to be held this weekend and US President Barack Obama and other world leaders were monitoring the cloud before confirming their presence.

The Norwegian and Belgian authorities said their airspace would remain closed most of Friday and even the outlook for the two days beyond was not optimistic.

Hundreds of flights out of London's Heathrow and Gatwick airports were cancelled, including transatlantic services. Britain extended its ban past midday Friday.

More than 200 flights into Brussels airport and more than 460 in an out of Spain, which was not covered by the cloud, fell victim to the cloud. Portugal cancelled 163 flights.

Scandinavian airline SAS said it had cancelled 635 flights alone.

Flights heading for Europe were stacked up all around the world. Norwegian Prime Minister Jens Stoltenberg was stranded in New York, the NTB news agency reported.

Amsterdam's Schiphol airport prepared beds and meals for stranded travellers. Hundreds spent the night at Brussels airport and others across northern Europe.

The National Air Traffic Services, which manages British airspace, said the cloud was moving south. "In line with international civil aviation policy, no flights other than agreed emergencies are currently permitted in UK controlled airspace," it said, grounding all non-emergency flights until 1200 GMT Friday.

The ash drifted at an altitude of about 5.0-6.0 miles (8.0-10 kilometers) and could not been seen from the ground. But experts said it posed a major threat.

In the past 20 years, there have been 80 recorded encounters between aircraft and volcanic clouds, causing the near-loss of two Boeing 747s with almost 500 people on board and damage to 20 other planes, experts said.

The prevailing winds allowed Icelandic airports to remain open.

The volcano on the Eyjafjallajokull glacier in southern Iceland erupted just after midnight on Wednesday.

Smoke from the top crater stacked more than 20,000 feet (6,000 metres) into the sky, meteorologists said. A 500-metre fissure appeared at the top of the crater on Wednesday, Iceland's RUV broadcaster said.

The heat melted the surrounding glacier, causing major flooding which forced the evacuation of between 700 and 800 people.

"We have two heavy floods coming out from the melting of the Eyjafjallajokull glacier," police spokesman Roegnvaldur Olafsson told AFP.

The eruption - in a remote area about 125 kilometres (75 miles) east of Reykjavik - was bigger than the blast at the nearby Fimmvorduhals volcano last month.

"It is very variable how long these eruptions last. Anywhere from a few days to over a year," Magnus Tumi Gudmundsson, a professor of geophysics and civil protection advisor in Iceland, told AFP.

"Judging from the intensity of this one, it could last a long time."

"There were more than 250 metres (820 feet) of thick ice on top of the crater. That quickly melted, causing massive flooding which caused some damage yesterday," Gudmundsson added.

Last month, the first volcano eruption at the Eyjafjallajokull glacier since 1823 - and Iceland's first since 2004 - briefly forced 600 people from their homes in the same area.

That eruption at the Fimmvorduhals volcano only ended on Tuesday, hours before the new one sent up the cloud.