The Icelandic Meteorological Office issued a red alert for air traffic on Saturday after a small lava eruption of the Bardarbunga volcano, beneath a glacier in the country's southeast.
Vulcanologist Melissa Pfeffer, who specializes in ash dispersion at Reykjavik headquarters, told the AP news agency that lava had melted ice underneath the Vatnajokull glacier.
However, she said it was not clear whether the eruption would ultimately send steam and ash into the air, as it depended on how much ice was melted.
"The thicker the ice, the more water there is, the more explosive it will be and the more ash-rich the eruption will be," she said.
Thousands of earthquakes have been registered deep beneath the glacier in the past week, with activity picking up on Saturday. The red alert issued by the Met office is the highest warning on the country's five-point scale.
Earlier this week, authorities ordered the evacuation of the highlands north of the Vatnajokull glacier, as a precautionary measure. The area is uninhabited but popular with hikers; hundreds were forced to leave.
A 2010 eruption of the Eyjafjallajokul volcano produced an ash cloud that caused a week of international aviation chaos, with more than 100,000 flights cancelled. Aviation regulators since have reformed policies about flying through ash, so a new eruption would be unlikely to cause disruption on that scale.
glb/jr (AP, dpa)