The ash plume spewed by Iceland's Eyjafjoell volcano has declined markedly in recent days and volcanic activity has "somewhat decreased," an expert told AFP Thursday, adding ash production could drop further.

The column of ash rising from Eyjafjoell has "hardly ever gone under five kilometres (3.1 miles) until yesterday," when it measured three kilometres, geophysicist Steinunn Jakobsdottir said.

"The column has been big and high for the past two weeks, but yesterday and today it's been considerably smaller," she added

In addition, "the (volcanic) activity has somewhat decreased," she said, adding it was "similar to what it was (during a lull) at the end of April, just before it peaked again in the first days of May."

Jakobsdottir stressed however that it was impossible to say when the eruption would stop.

"But if we don't get another session of earthquakes and (if) magma flows, then ash production will drop," she noted.

Iceland's Eyjafjoell volcano, which first blew on April 14, briefly transformed into a non-explosive eruption with tranquil lava flows at the end of April before again becoming an explosive, ash-spewing eruption.

It has peaked three times, with the towering column of ash initially emitted blowing into European skies and causing the biggest shutdown of the continent's airspace since World War II.

Two smaller peaks in May have also caused disruptions in European skies.