Smoke billows from a volcano in Eyjafjallajokull on Wednesday following a powerful eruption. The volcano is buried under a glacier about 120 kilometres east of Reykjavik, Iceland (Ingolfur Juliusson/Reuters)

A volcano under a glacier in Iceland erupted Wednesday, melting ice, shooting smoke and steam into the air and forcing hundreds of people to leave their homes.

Authorities moved 800 residents away from the Eyjafjallajokull glacier as rivers rose by up to three metres. Rognvaldur Olafsson, a chief inspector for the Icelandic Civil Protection Agency, said no lives or properties were in immediate danger.

Emergency officials and scientists said the eruption under the ice cap was 10 to 20 times more powerful than an eruption that happened in the area late last month, and carried a much greater risk of widespread flooding.

"This is a very much more violent eruption because it's interacting with ice and water," said Andy Russell, an expert in glacial flooding at the University of Newcastle in northern England.

"It becomes much more explosive, instead of a nice lava flow oozing out of the ground."

The volcano, about 120 kilometres east of Reykjavik, erupted March 20 after almost 200 years of silence.

Pall Einarsson, a geophysicist at the University of Iceland, said magma was melting a hole in the thick ice covering the volcano's crater, sending water coursing down the glacier.

Iceland's main coastal ring road was closed near the volcano, and workers smashed a hole in the highway in a bid to give the rushing water a clear route to the coast and prevent a major bridge from being swept away.

No signs of volcanic ash

Scientists aboard a coast guard plane that flew over the volcano said the new fissure appeared to be up to two kilometres long.

An emergency worker talks to a driver after hundreds of residents were forced to move away from the volcano. (Brynjar Gauti/Associated Press)There were no immediate signs of large clouds of volcanic ash, which could disrupt air travel between Europe and North America. Some domestic flights were cancelled, but Iceland's international airport remained open.

The original eruption petered out earlier this week. But Gunnar Gudmunsson, a geophysicist at the Icelandic Meteorological Office, said there had been a series of tremors overnight and rivers in the area began rising Wednesday morning — strong evidence of a new eruption under the glacier.

"Most probably this eruption is taking place at the summit ... under the ice," he said.

Last month's eruption struck near the glacier in an area that had no ice. Gudmunsson said the new eruption appeared to be about eight or nine kilometres west of the original fissure.

Officials monitor larger volcano

Iceland, a country of 320,000 people, sits on a large volcanic hot spot in the Atlantic's mid-oceanic ridge. Volcanic eruptions are often triggered by seismic activity when the Earth's plates move and when magma from deep underground pushes its way to the surface.

Scientists said they are monitoring another volcano, Katla, which in the past has erupted in tandem with Eyjafjallajokull.

Katla is under the vast Myrdalsjokull ice cap. Its last major eruption took place in 1918 and experts believe a new blast is overdue.

"So far, there have been no signs of the reawakening of the Katla volcano, but a lot of things can still happen, so we are monitoring it quite closely," Einarsson said.

With files from The Associated Press