Hundreds of flights worldwide were cancelled Thursday as volcanic ash from Iceland swept across northern Europe, causing eight countries to close their airspace amid warnings of days of disruption.

Thousands of passengers from Hong Kong to Dublin were stranded as aviation chiefs decided it was too risky to send planes through the cloud of ash.

The volcanic eruption under the Eyjafjallajokull glacier Wednesday spewed out ash which blew towards Norway and Scotland, and by Thursday had drifted over Britain, Ireland, Scandanavia, Belgium, the Netherlands and France.

Although not visible from the ground, volcanic ash can be highly dangerous for aircraft, clogging up the engines and reducing visibility, experts warn.

Flights into northern Europe from cities including Tokyo, Hong Kong, Mexico City, Dubai, New York and Paris were cancelled or subject to lengthy delays.

Norway was the first to ground its flights on Wednesday evening, and a wave of flight cancellations followed before air traffic controllers decided to close British and Irish airspace to all non-emergency flights from 1100 GMT.

"NATS advises that these restrictions will remain in place in UK-controlled airspace until 0700 tomorrow, Friday 16 April, at the earliest," said the National Air Traffic Services, which manages British airspace.

The Irish Aviation Authority said its restrictions would remain until 10:00 am (0900 GMT) "at the earliest".

Norway, Sweden and Denmark also closed their airspace, Finland halted most air traffic - although Helsinki airport remained open - and Belgian, Dutch and French airspace was progressively shut down during the day.

In France Charles de Gaulle and Orly, the capital's two main airports, were to shut by 2100 GMT at the latest, while other airports in the north of the country started closing at 1500 GMT, the DGAC aviation authority said.

Norwegian Transport Minister Magnhild Meltveit Kleppa said the "airspace will be closed tomorrow too, and the outlook for the next two-three days is not good".

The closure left Prime Minister Jens Stoltenberg stranded in New York and Crown Prince Haakon stuck in London, according to the NTB news agency, while regional airline SAS said it had cancelled three-quarters of its flights.

Icelandic airports, however, reported no problems.

"The wind is blowing the ash to the east," Hjordis Gudmundsdottir of the Icelandic Airport Authority told AFP, adding: "It's amazing really."

About 300 flights in and out of London's Heathrow and Gatwick airports had already been cancelled before the airspace was closed, leaving many of the 260,000 passengers that typically use the airports each day with nowhere to go.

"Basically we're stranded here, and a lot of people are angry. I realise it's an act of God - however it would be nice to have another exit strategy," said Isobel Connolly, who was due to fly from Heathrow to Ireland.

US airlines said many flights to Britain had been cancelled, as were some flights that connected there, and they were braced for more disruption.

A NATS spokeswoman said the closure of British airspace could affect flights from other parts of Europe to the United States.

"A lot of traffic from Western Europe to America would normally fly through our space," she told AFP, adding: "They'll probably be re-routed."

Some airlines raced to get their flights completed before the cloud hit, diverting planes intended for northern England to London before it closed, or sending those intended for London to Frankfurt in Germany.

David Rothery, a senior lecturer in earth sciences at Britain's Open University, said flight restrictions were an essential safety precaution.

"This is because if volcanic ash particles are ingested into a jet engine, they accumulate and clog the engines with molten glass," he said.

In 1982, British Airways and Singapore Airways jumbo jets lost their engines when they flew into an ash cloud over Indonesia, while a KLM flight had a similar experience in 1989 over Alaska.

"On each occasion, the plane fell to within a few thousand feet of the ground before it was possible to restart the engines," Rothery said.