A small volcanic eruption has occurred under Iceland's Dyngjujokull glacier, prompting authorities to raise the warning code for aviation to red, the highest level, Iceland's meteorological office said.

The region, in the centre of the North Atlantic island nation, has already been evacuated due to days of heightened seismic activity there.

"It is believed that a small subglacial lava-eruption has begun under the Dyngjujokull glacier," the Icelandic Met Office said. "The aviation color code for the Bardarbunga volcano has been changed from orange to red."

Ash from the eruption of Iceland's Eyjafjallajokull volcano in 2010 shut down much of Europe's airspace for six days.

The red code indicates that an eruption is imminent or underway with a significant emission of ash likely.

Brussels-based aviation authority Eurocontrol said that as soon as the volcano had erupted, the Volcanic Ash Advisory Centre in London would produce a regular forecast about the levels of volcanic ash in the atmosphere.

Based on this forecast, civil aviation authorities may issue a notice but it was the responsibility of individual airlines whether they would operate and how they would adapt their flight schedules, Eurocontrol said.