High altitude winds were pushing the ash cloud directly south over the north Atlantic at a height of about nine kilometers with little impact on flight patterns in northern Europe, except in parts of Ireland, the scientists said.
"The effect on European air traffic should be negligible but that, of course, is up to aviation authorities in each country," meteorologist Gudrun Nina Peterson told a news conference.
The ash cloud rising from the Eyjafjallajokull volcano, located beneath a glacier in southern Iceland, disrupted air travel over northern Europe for a week last month, robbing cash-starved airlines of billions of euros in revenue.
The cloud had shot up as high as 10 kilometers into the air on Tuesday, higher than during the crisis last month, before subsiding somewhat on Wednesday, Peterson said.
Experts said they had picked up signs of increasing seismic activity deep under the volcano which indicated that magma was forcing its way up through layers of rock and approaching the volcanic crust.
"The eruption is going at full speed," said geophysicist Sigurlaug Hjaltadottir. "This is evident by our measurements and by the fact that there is still quite a bit of ash production in the volcano, coupled with massive explosions."
So far the eruption has produced little lava and large amounts of ash -- the result of a fragmentation process by which magma breaks down on its way to the surface under pressure of expanding gases to form tiny, abrasive particles.
If absorbed into the engines of an airplane, the ash particles can cause serious damage.
Hjaltadottir added that ash was still falling over the sparsely populated farm land around the volcano, some 120 kilometers southeast of Reykjavik, but at a far lower rate than at the start of the eruption.
Earlier on Thursday, Ireland lifted all restrictions on its airports on Thursday after a volcanic ash cloud blew away from Europe after having disrupted flights for days.