Air travel across much of Europe was paralyzed for a fourth day on Sunday because of a huge cloud of volcanic ash, but Dutch and German test flights carried out without apparent damage seemed to offer hope.
Many countries closed their airspaces until well into Sunday or Monday, leaving tens of thousands of passengers stranded worldwide, and weather experts said wind patterns meant the cloud was not likely to move far until later in the week.
They said the plume floating through the upper atmosphere from Iceland could become more concentrated on Tuesday and Wednesday, posing an even greater risk and threatening to compound airline losses running at more than $200 million a day.
The no-fly rulings have been imposed because the dust of pulverized rock and glass particles can paralyze jet engines and damage airframes but the test flights on Saturday prompted some optimism from airline officials.
Dutch airline KLM said it flew a Boeing 737-800 at the regular altitude of 10 kilometers (6 miles) and up to the 13 km maximum. Germany's Lufthansa said it flew 10 planes to Frankfurt from Munich at altitudes of up to 8 km.
"We have found nothing unusual, neither during the flight, nor during the first inspection on the ground," KLM chief executive Peter Hartman, who took part in his airline's test, said in a statement.
"If the technical examination confirms this image, we are ready tomorrow to fly back our seven planes from Duesseldorf to Amsterdam. We then hope to get permission as soon as possible to partially restart our operations."
Dutch officials said more test flights would take place on Sunday.
The air travel disruption is the worst since the September 11 attacks on New York and Washington in 2001, when US airspace was closed for three days and European airlines were forced to halt all transatlantic services.
European aviation agency Eurocontrol said no landings or takeoffs had been possible for civilian aircraft in most of northern and central Europe on Saturday because of the ash spewed out by the Icelandic volcano, which was still erupting.
It expected 5,000 flights in European airspace on Saturday, compared with the usual 22,000. On Friday there were 10,400 flights compared with the usual 28,000, said the agency.
The cloud has forced several world leaders to rearrange travel plans. US President Barack Obama, German Chancellor Angela Merkel and others have canceled trips to Poland for the funeral on Sunday of Polish President Lech Kaczynski, killed in a plane crash in Russia a week ago.
The volcanic eruption appeared to be easing on Saturday but could go on for days or even months, officials said.
US-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," said AccuWeather.
It said an Atlantic storm and change in the direction of the jetstream on Thursday could break up the cloud.
Britain's weather agency told BBC television it was likely the cloud would remain over Britain for some days.
Britain, Germany and Denmark were among the countries to announce their airspaces were closed for the whole of Saturday. Early on Sunday, Britain and Germany extended their shutdowns until 2 p.m. ET.
France said Paris airports would be closed until at least Monday morning. Italy maintained a shutdown of its northern airports. The Netherlands and Switzerland extended their no-fly rulings until 8 a.m. ET on Sunday.
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," said IHS Global Insight chief UK and European economist Howard Archer.
Kenya's flower exporters said they were already losing up to $2 million a day because they had not been able to airlift their blooms. Kenya accounts for about a third of flower imports into the European Union.
Airlines could also suffer a severe financial blow.
British Airways, hit by strikes last month that cost it around $70 million, canceled all Sunday's flights.
Ireland's Ryanair, Europe's biggest low-cost carrier, has canceled all flights to and from northern European countries until 1200 GMT on Monday.
Europe's biggest tour operator, TUI Travel, said it was cancelling all trips until at least 0800 GMT on Sunday.
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.
Disruption spread to Asia, where dozens of Europe-bound flights were canceled and hotels from Beijing to Singapore strained to accommodate stranded passengers. In Singapore, 45 flights were canceled on Saturday, Changi Airport said.
More than four in five flights by US airlines to and from Europe were canceled on Saturday. Shipping company FedEx Corp said more than 100 FedEx Express flights headed to Europe were rerouted, diverted or canceled within the past 72 hours.
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."