Two earthquakes measuring over 5 in magnitude - the biggest yet - shook Iceland's Bardarbunga volcano Sunday after the country issued an aviation red alert warning that an ash-emitting eruption may be imminent, the Associated Press reported.
Iceland's Meteorological Office recorded earthquakes of 5.3 and 5.1 in the early hours. It said there were no signs of an eruption.
The volcano, underneath Iceland's vast Vatnajokull glacier, has been rattled by thousands of small earthquakes over the past week.
On Saturday scientists reported a small eruption under the ice, but it was not visible on the surface, and the Met Office later said it appeared to have stopped.
Authorities have declared a no-fly zone of 100 nautical miles by 140 nautical miles (185 kilometers by 260 kilometers) around the epicenter as a precaution.
A 2010 eruption of Iceland's Eyjafjallajokul volcano caused a week of international aviation chaos, with more than 100,000 flights cancelled. Aviation officials closed Europe's air space for five days out of fear that volcanic ash could harm jet engines.
Any new eruption is likely to be less disruptive. European aviation authorities have changed their policy, giving airlines detailed information about the location and density of ash clouds but leaving decisions to airlines and national regulators.
Britain's National Air Traffic Service said it was monitoring what it called a "dynamic situation" but was expecting normal operations Sunday.
The agency said it was using alternative flight paths across the Atlantic as a precaution.
Virgin Atlantic said a London-to-San Francisco flight took a slight detour Saturday to avoid Iceland's no-fly zone.