OSLO (AFP) - Just as flights gradually began resuming across much of northern Europe Monday, meteorologists warned that winds carrying a massive cloud of volcanic ash from Iceland would likely shift back towards Scandinavia in coming days.

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"The conditions now (in northern Europe) are fairly good, but ... in the course of the day tomorrow (Tuesday), the situation will worsen when it comes to ash, soot and particles from the volcano" in this region, Haakon Melhuus, a meteorologist at the Norwegian Meteorological Institute said.

"There will probably be fairly large concentrations of dust and such coming from (Iceland) across southern Scandinavia and eastward to Finland," he told AFP.

His comments came just as airports across much of Norway, Sweden and Finland slowly began re-opening for business, to the relief of millions of passengers stranded around the world waiting for flights to and from Europe.

Denmark however continued to push back its deadline for opening airports, although it did open airspace above 35,500 feet (10,670 metres) to allow planes to fly over to open airports further north.

But the openings to the north could be brief, Melhus warned.

"Tomorrow morning, we will have a situation that might be similar to the one we had before the weekend," when flights across the entire Nordic region, with the ironic exception of Iceland, were grounded, he said.

The Finnish Meteorological Institute also said air "currents are shifting the ash cloud from Iceland towards Scandinavia via Great Britain and Germany."

"The ash content is temporarily lower around (Finland and Scandinavia Monday), but according to forecasts a new, denser ash cloud will arrive from the west during the day Tuesday," it said in a statement.

There was however still hope for European air traffic, experts said, pointing out Monday that the second eruption at the Eyjafjoell volcano in Iceland in less than a month was diminishing, as was the column of ash it had been spewing since Wednesday.

A smaller plume would likely mean the ash would not be swept away by the strongest high altitude winds and would not be transported as far, Melhus said.

The column produced a cloud of ash that was transported to Europe by high level winds, causing the shutdown of airspace across a wide swath of the continent, because ash can damage aircraft engines.