A huge cloud of volcanic ash from Iceland spread over half of Europe on Friday, forcing the cancellation of thousands more flights in the continent's biggest air travel shutdown since World War II.

Europe's air traffic control centre predicted 17,000 flights would be cancelled Friday. And as the giant no-fly zone grew, Poland said it may delay the funeral of President Lech Kaczynski on Sunday because of the cloud threat.

Experts warned the fallout from the Eyjafjallajokull volcano in southeast Iceland could take several days to clear and aviation authorities refused to say when the skies would clear again.

The cloud now extends from the Atlantic to the Russian capital and from the Arctic Circle to Austria. Thousands of people were stranded in airports around the world as a global flight backlog built up.

All of Europe's three biggest airports - London Heathrow, Paris-Charles de Gaulle and Frankfurt - were closed by the ash, which is a threat to jet engines and pilot visibility.

Eurocontrol, the European air traffic control group, said only 11,000 of the daily 28,000 flights in the affected zone would take off Friday. It said at least half of the 600 daily flights between Europe and North America would be cancelled.

About 6,000 flights to and within Europe were cancelled Thursday.

Poland, Britain, Austria, the Czech Republic, Denmark, Lithuania, Latvia, Estonia, Belgium and the Netherlands shut down all or most of their airspace.

Finland, France, Germany, Russia and Spain experienced major disruption, although Sweden began gradually reopening airspace and Norway temporarily opened up some of its.

"Forecasts suggest that the cloud of volcanic ash is continuing to move east and southeast and that the impact will continue for at least the next 24 hours," Eurocontrol said in a statement.

Most aviation authorities promised a review on Friday, but the Dutch transport inspectorate set the uncertain tone: no flights "until further notice".

Delaying Kaczynski's funeral is a "serious alternative" due to the closure of Polish airspace, a presidential official has said.

US President Barack Obama and other world leaders are expected at the ceremony in the southern Polish city of Krakow but all are now monitoring the cloud before confirming their attendance.

In Britain, airports including London Heathrow, the world's busiest international air hub, were deserted as operators warned travellers not even to turn up for booked flights.

Officials extended the ban on non-emergency flights in most of its airspace until 0000 GMT Saturday "at the earliest", although some flights in Northern Ireland and western Scotland will be allowed.

Debbie Eidsforth, 36, spent the night at Heathrow and was trying to get back to Adelaide in Australia via Hong Kong.

"I had paid 5,500 pounds (6,300 euros, 8,500 dollars) for my flights, but it doesn't matter what class you fly in, everyone's in the same situation," she said.

"I just slept here on the seats, and there were quite a few other people dotted around. They should really have bought blankets and coffee around for us."

In Scotland, health officials warned that ash falling to the ground over northern Britain might cause symptoms such as itchy eyes or a sore throat.

Amsterdam's Schiphol airport prepared beds and meals for 2,000 stranded travellers. Hundreds spent the night at the Brussels airport and others across northern Europe.

The prevailing winds, however, allowed Icelandic airports to remain open.

The ash drifted at an altitude of about 8.0-10 kilometres (5.0-6.0 miles). Although it could not been seen from the ground, 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 volcano on the Eyjafjallajokull glacier in southern Iceland erupted just after midnight on Wednesday.

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

The heat melted the surrounding glacier, causing major flooding that forced the evacuation of about 800 people for a second time on Thursday.

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.

Some experts have warned that the eruption could last up to a year.