France's transport minister, Dominique Bussereau, said there will be a meeting on Monday of European ministers affected by the crisis to coordinate efforts to reopen airspace.

Meteorologists warned that the situation above Europe was constantly changing because of varying winds and the continuing, irregular eruptions from the Icelandic volcano. That uncertainty is bumping up against Europe's need to resume flights.

"There is currently no consensus as to what consists an acceptable level of ash in the atmosphere," said Daniel Hoeltgen, a spokesman for the European Aviation Safety Agency. "This is what we are concerned about and this is what we want to bring about so that we can start operating aircraft again in Europe."

The ranks of stranded passengers, meanwhile, were growing, and many would be stuck for days even if restrictions were fully and immediately lifted.

Mike Parker, trying to return to London from Milan, had made it as far as Paris by Sunday. He stood in line for hours for a bus ticket, only to be told "there's nothing available, there's no trains, there's no planes, there's nothing."

"This is my third day and it looks like it might be another three days before I get back," Parker said.

Rognvaldur Olafsson, a spokesman with the Civil Protection Agency in Iceland, said Sunday the eruption is continuing and there are no signs that the ash cloud is thinning or dissipating.

"It's the same as before," he said. "We're watching it closely and monitoring it."

German air traffic control was the first on Sunday to loosen its ban on passenger flights, allowing some traffic from Frankfurt and airports in the north, but only for northern destinations. Eastward-bound flights were permitted from Berlin, Hannover, Erfurt and Leipzig. The Swiss Federal Office of Civil Aviation also began allowing some flights Saturday.

AccuWeather.com said the top of the ash plume had dropped to about 10,000 feet from 33,000 earlier in the week, putting it in the flight path of even low-flying aircraft. Shifts in the wind will increase the risk for the Netherlands and Germany on Tuesday and Wednesday, the forecaster said.

Ash and grit from volcanic eruptions can damage a plane in various ways. The abrasive ash can sandblast a jet's windshield, block fuel nozzles, contaminate the oil system and electronics and plug the tubes that sense air speed. The greatest danger is to the engines, where melted ash can then congeal on the blades and block the normal flow of air.

There are no recorded instances of fatal aircraft crashes involving volcanic ash, though several have suffered damage and some temporarily lost engine power.

Scientists say that because the volcano is situated below a glacial ice cap, magma is being cooled quickly, causing explosions and plumes of grit that can be catastrophic to plane engines.

"Normally, a volcano spews out ash to begin with and then it changes into lava, but here it continues to spew out ash, because of the glacier," said Reynir Bodvarsson, director of Swedish National Seismic Network.

Bodvarsson said the relative weakness of the eruption in Iceland also means the ash remains relatively close to the earth, while a stronger eruption would have catapulted the ash out of the atmosphere.