* Worst disruption since Sept. 11 attacks
* Nearly three in four European flights grounded
* Chaos costing airlines hundreds of millions of dollars
(Updates with extended closures, weather experts, Obama)
By Mark Trevelyan
LONDON, April 17 (Reuters) - Europe's air travel chaos deepened on Saturday as a huge cloud of volcanic ash spread southeast across the continent, halting more than three in four flights and stranding tens of thousands of passengers worldwide.
European aviation agency Eurocontrol said no landings or takeoffs were possible for civilian aircraft in most of northern and central Europe because of the ash spewed out by the Icelandic volcano, which was still erupting.
Many countries closed airspace until Sunday or Monday and weather experts forecast the cloud would not move far. They said the plume of ash could even become more concentrated on Tuesday and Wednesday, posing a greater threat to air travel.
The plume that floated through the upper atmosphere, where it could wreak havoc on jet engines and airframes, is costing airlines more than $200 million a day.
Test flights without passengers are taking place in the Netherlands and other European countries to assess the impact of the ash on aircraft, the Dutch government said on Saturday.
For factbox on national airspace closed by ash cloud,
double-click on [nLDE63F11N]
For map of ash cloud movement and airline routes, click:
For more stories on volcano's impact, click on [nLDE63F189]
The disruption is the worst since the Sept. 11 attacks on New York and Washington in 2001, when U.S. airspace was closed for three days and European airlines were forced to halt all transatlantic services.
Eurocontrol expected 5,000 flights in European airspace, compared with 22,000 on a normal Saturday. On Friday it said there were 10,400 flights compared with the usual 28,000.
"Forecasts suggest that the cloud of volcanic ash will persist and that the impact will continue for at least the next 24 hours," the agency said in a statement just after 0900 GMT.
The cloud has forced several world leaders to hastily rearrange travel plans. U.S. President Barack Obama and German Chancellor Angela Merkel and others have cancelled trips to Poland for the funeral on Sunday of Polish President Lech Kaczynksi, killed in a plane crash in Russia a week ago.
The volcanic eruption appeared to be easing on Saturday but could continue for days or even months to come, officials said. [ID:nLDE63G0DE].
U.S.-based forecaster AccuWeather said the ash was in an area of weak wind flow and was unlikely to move far on Monday.
"The plume is expected to become more concentrated Tuesday and Wednesday, posing a greater threat to air travel. However, it is also expected to become narrower, impacting a smaller area," it said, adding an Atlantic storm and change in the direction of the jetstream on Thursday could break up the cloud.
"A southwest jet stream should help clean any remaining ash plume out of most of Europe and bring some relief for the aviation crisis," it said.
Britain's weather agency told BBC television it was likely the cloud would remain over Britain for some days.
"The ash hangs around for several days once it has got here. So we need a change of wind direction that stays changed for several days. And there is no sign of that in the immediate future," Brian Golding, head of Met Office forecasting, said.
Britain, Denmark and Germany were among the countries to announce their airspace was closed for the whole of Saturday and German carrier Lufthansa (LHAG.DE) said it had no planes in the air anywhere in the world.
However, unless the cloud disrupts flights for weeks, threatening factories' supply chains, economists do not think it will significantly slow Europe's shaky recovery from recession or affect second-quarter gross domestic product figures.
"The overall impact should be very limited even if the problem persists for a day or more," IHS Global Insight chief UK and European economist Howard Archer said. [ID:nLDE63F1NL]
Airlines, however, could suffer a severe financial blow.
France said Paris airports would remain closed until at least Monday morning. Italy extended a shutdown of its northern airports until then. The Dutch extended a shutdown to Sunday morning and Switzerland closed airports until 1200 GMT Sunday.
Europe's biggest tour operator, TUI Travel TT.L, said it was cancelling all trips until at least 0800 GMT on Sunday. British Airways BAY.L, hit by strikes last month that cost it around $70 million, cancelled all Sunday's flights.
Ireland's Ryanair (RYA.I), Europe's biggest low-cost carrier, has cancelled all flights to and from northern European countries until 1200 GMT on Monday.
The fallout hit airline shares on Friday with Lufthansa, British Airways, Air Berlin, Air France-KLM, Iberia and Ryanair down between 1.4 and 3.0 percent. [ID:nLDE63F07I].
Disruption spread to Asia, where dozens of Europe-bound flights were cancelled and hotels from Beijing to Singapore strained to accommodate stranded passengers. In Singapore, 45 flights were cancelled on Saturday, Changi Airport said.
An official at Eurocontrol said the number of transatlantic flights arriving in Europe was only one third to a quarter of the usual number for a Saturday.
American Airlines, a unit of AMR Corp AMR.N, said it was able to operate flights to and from Spain and Italy but had cancelled 56 others to and from Europe, the same as on Friday.
The U.S. military had to re-route many flights, including those evacuating the wounded from Afghanistan and Iraq, a Pentagon spokesman said.
The volcano began erupting on Wednesday for the second time in a month from below the Eyjafjallajokull glacier, hurling a plume of ash 6 to 11 km (3.7 to 6.9 miles) into the atmosphere.
By Saturday this had fallen to 5 to 8 km (3 to 5 miles).
"The eruption could go on like that for a long time," said Bergthora Thorbjarnardottir, a geophysicist at the Meteorological Office. "Every volcano is different and we don't have much experience with this one -- it's been 200 years since it erupted last." (Reporting by London, Geneva, Dublin, Paris, Amsterdam, Brussels, Reykjavik, Washington, Frankfurt and Berlin newsrooms; Writing by Mark Trevelyan; editing by Alison Williams)