A massive flood of meltwater poured out of Iceland's Myrdalsjoekull glacier Saturday, raising fears of an eruption from the powerful Katla volcano underneath, but experts said a large blast was unlikely.

"At around 3:00 am (0300 GMT) ... we had a glacial meltwater runoff from underneath the glacier," Evgenia Ilyinskaya, a volcanologist at the Icelandic Meteorological Office, told AFP.

The giant flood took out a bridge and part of the main road that circles Iceland and prompted evacuations, but there were no reports of injuries.

The flooding sparked fears of an eruption at Katla, known to be one of Iceland's most powerful volcanoes and located just southeast of Eyjafjoell, notorious for last year's flight-halting gigantic ash cloud.

But experts said geothermal heat, and not an eruption, might be the culprit behind the flow of meltwater.

"There is always meltwater because there is geothermal heat under the glacier. So the meltwater builds up and finally floods down," Ilyinskaya pointed out.

"While we can't say for sure there was not a small subglacial eruption that caused it, we don't see any signs of it coming up to the surface," she said.

And according to Freysteinn Sigmundsson, a geophysicist at the University of Iceland, the floods had begun subsiding by Saturday afternoon.

"There are two possibilities: it is a volcanic eruption, and then its seems to be a small one, or it is only geothermal water," he told AFP.

If there had in fact been a small subglacial eruption, he said, "this doesn't seem to be the large eruption that people have been waiting for at Katla".

Katla, which is named after an Icelandic witch and said to be 10 times more powerful than Eyjafjoell, last erupted in 1918, sending a wall of meltwater down the glacier, bearing ice chunks the size of houses, and blanketing southern Iceland in thick ash.

And according to experts, the volcano, which is also located to the southeast of Grimsvoetn which was behind Iceland's latest eruption in May, is overdue for a powerful blast.

The seismometres had meanwhile gone crazy at the time the so-called river-run occurred, and Ilyinskaya hinted that the activity at Katla may actually have caused sensors around the Hekla volcano, about 110 kilometres (70 miles) east of Reykjavik, to indicate in recent days an eruption there could be imminent.

"It's not unlikely that the activity we saw there was actually caused by the Katla glacier. They're pretty close," she said, adding that measurements around both volcanoes now seemed calm.

Regardless of what caused Saturday's flood, the enormous river gushing from the glacier had taken out a 128-metre-long bridge and part of the road and prompted authorities to evacuate hundreds of people from surrounding areas due to the continued danger of flash floods.

The Icelandic Red Cross set up evacuation centres in the villages of Vik and Kirkjubaejarklaustur which in total had taken in more than 200 residents and tourists.

"Travelers are safe in Iceland except in the defined danger zone at and around the Myrdalsjoekull glacier," Iris Marelsdottir, a project manager at the Department of Civil Protection, told AFP.

She stressed however that people in that area needed to "get away from there as soon as possible".

Hreinn Haraldsson, who heads Iceland's Road Administration, meanwhile said it would take two to three weeks to build a preliminary bridge to replace the one washed away.

"The crack in the highway is of course a big concern for the general traffic and tourism in Iceland," he told AFP.