The estimated 1-km fissure eruption in the Holuhraun area prompted the Icelandic Met Office to raise the warning code for aviation to red, the highest level.
The rumblings at Iceland's largest volcano system, which is covered by a several hundred meters thick glacier, have raised worries of an eruption that could spell trouble for air travel. In 2010, an ash cloud from the Eyjafjallajokull volcano closed much of Europe's airspace for six days.
The eruption just after 0000 GMT took place in a lava field not covered by ice. The risk of an ash cloud is highest in case of a sub-glacial eruption.
"The Icelandic Met Office has raised the aviation colorcode over the eruption site to red and the Icelandic Air Traffic Control has closed down the airspace from the earth up to 18000 feet," Iceland's National Crisis Coordination Centre said.
"No volcanic ash has been detected with the radar system at the moment....Seismic eruption tremor is low indicating effusive eruption without significant explosive activity."
Icelandic Met Office spokeswoman Kristin Gudmundsdottir said activity at the fissure seemed to be unchanged in the morning hours after decreasing two hours after the eruption started.
The red warning code indicates that an eruption is imminent or underway, with a risk of ash.
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