After flying over the site, seismologist Ragnar Stefansson, from the Icelandic Meteorological Office, said: "The eruption seems to be following a typical pattern and is now producing less smoke but more surface volcanic matter.

Clare Lyons reports on a dramatic eruption

"But we are still detecting continuous volcanic tremors from the eruption site and can expect activities to last for a few days yet."

The eruption began on Friday under the Vatnajokull glacier, which covers eight percent of Iceland's 103,000km (64,000 square mile) area. Scientists said it would not endanger populated areas or threaten a major flood.

In the last eruption in a more northern part of Vatnajokull in 1996, melted ice poured into the lake, spilling over and flooding the uninhabited black sands on Iceland's south coast. Roads and bridges were swept away.

"The volcanic matter seems to be going into the lake under the glacier, which could precipitate an outflow but not on any big scale," Stefansson added.

Police have mounted a watch at both ends of the road across the sands, ready to close it if a flood starts.

The Civil Defence Authority has banned all movement on the glacier within a 10km (six mile) radius of the eruption but has not issued any general alert.

The billowing grey volcanic cloud was visible in parts of the capital Reykjavik, 250km (160 miles) away, where two-thirds of Iceland's 275,000 people live.

Its height has dropped between six and eight km (four to five miles) and no ash has been detected in the farmland and fishing villages south-east of the glacier.

Iceland is one of the most active volcanic areas in the world, with an eruption every five years over the past 1100 years.

Records show that over the centuries, the volcanoes around Grimsvotn alternate in roughly 60-year cycles of frequent eruptions and relatively quiet phases.

Scientists say the two eruptions in the past three years could signal that a new active phase has begun.

Grimsvotn means "the lakes of Grim" (a man's name) and dates from the time when lakes separated the glaciers on what is now a continuous ice cap, before the "little ice age" from 1400 to the 19th century when the earth's average temperatures dropped.