Air connections between some major US cities and Britain and points in Europe were cancelled Thursday after a volcano in Iceland spewed huge clouds of ash into the skies above northern Europe.

"There are no flights that are leaving or coming from the United Kingdom," said Steve Coleman, a spokesman for the Port Authority of New York and New Jersey, which oversees JFK, LaGuardia and Newark airports.

The intergovernmental Eurocontrol agency said half of all transatlantic flights are expected to be cancelled on Friday due to the volcanic ash. An average of 600 flights take place each day between Europe and North America.

Overall, 5,000 to 6,000 flights were likely cancelled on Thursday, with the airspace of Britain, Ireland, Belgium, the Netherlands, Denmark, Sweden, Norway, Finland, the north of France, parts of northern Germany and part of northern Poland closed to civilian aircraft, according to Eurocontrol.

Thousands of flights were grounded as countries imposed the biggest airspace closure since the September 11 attacks in 2001, while experts warned that fallout from the Eyjafjallajokull volcano could take several days to clear.

The International Air Transport Association (IATA) said it was coordinating with Britain's main air navigation services provider, National Air Traffic Services (NATS) "on implementing contingency measures to ensure that flights are re-routed through adjacent airspace" to avoid the ash plumes.

US airlines scrambled to come up with alternatives as thousands of customers were forced to delay their flight plans.

United Airlines said "we are experiencing major disruption to our services and timetable" at London Heathrow airport, and noted the situation was evolving after NATS restricted flights within affected areas.

"This is an evolving situation," it added, saying it had issued travel waivers for its customers to change their travel plans free of charge.

Continental Airlines cancelled at least 32 flights and said that its Newark hub was severely affected, as well as some flights from its Houston, Texas hub.

It allowed refunds and waived change fees and fare differences for customers with travel starting by May 2. For other dates, change fees were waved although fare differences may apply.

And US Airways said it had cancelled six flights so far from Philadelphia to Manchester, Heathrow and Dublin.

Delta Air Lines cancelled 65 international flights - 35 of them late Thursday and the rest for early Friday - from its US hubs to Paris, Amsterdam, London, Manchester, Dublin, Shannon, Brussels and Bombay.

All Delta flights to British airports and Paris's Charles de Gaulle airport were cancelled for Thursday and early Friday.

It allowed customers ticketed to travel from or through affected areas Thursday through Sunday could make a one-time change to their flight plans without fees so long as tickets are changed by April 30. Customers whose flights were cancelled were also allowed to request refunds.

Airports serving Washington were running normally but bracing for disruption.

"There will be (delays). Most of our international flights from that part of the world are in the afternoon," said Courtney Mickalonis, spokeswoman for the Metropolitan Washington Transit Authority.

At nearby Baltimore airport, the one direct daily flight to Europe, operated by British Airways, was cancelled.

"It very much depends on what happens with the volcano" as to whether there would be further delays, airport spokesman Jonathan Dean told AFP.

Florida airports felt little impact from the disruptions in Europe.

Orlando International Airport spokesman Rod Johnson said there had only been two flight cancellations due to the volcanic ash.

Miami Airport reported that one of two British Airways flights due Thursday had been cancelled and further delays were possible when flights arrive from Europe and return.

A spokesman for Delta Airlines at Atlanta International Airport said British-connected flights had been shut down.

Chicago's O'Hare airport said 20 international flights to European destinations had been canceled so far, while Midway international airport reported normal operations.