Iceland has issued a red alert to aviation after indications of a possible eruption under the country's biggest glacier, the Vattnajokull.
The Icelandic Met Office warned that a small eruption had taken place under the Dyngjujokull ice cap.
Seismic activity is continuing at the Bardarbunga volcano, about 30km away.
Airspace over the site has been closed, but all Icelandic airports currently remain open, authorities say. A Europe-wide alert has also been upgraded.
European air safety agency Eurocontrol said it would produce a forecast of likely ash behaviour every six hours.
Iceland's Eyjafjallajokull volcano erupted in 2010, producing ash that severely disrupted air travel.
The red alert is the highest warning on the country's five-point scale.