Two earthquakes measuring more than magnitude 5 hit Iceland's Bardarbunga volcano overnight and another quake shook a nearby volcano, with overall seismic activity staying high, the country's Meteorological Office has said.
The rumblings at Iceland's largest volcano system 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.
"During the night we have had three larger events, two of them in the Bardarbunga caldera. Those were 5.2 and 5.3, and very similar to the events that we have seen there before," Palmi Erlendsson, a geologist at the Met Office, said.
He said there had also been a 4.5 magnitude quake at the Askja volcano, 50km to the north, probably because of magma from Bardarbunga moving in that direction.
The magma still has 15km to go before potentially reaching Askja, a scenario which could also cause an ash-producing eruption, like an eruption at Bardarbunga itself, Mr Erlendsson said.
The most likely scenario if there were an eruption now, would be one between the two volcanoes, he said.
"That would not be as dangerous ... in terms of ash," he said. "But it's impossible to say what will happen," he added, pointing to the possibility of magma continuing to flow toward Askja, or even of the volcanic activity halting altogether.
A magnitude 5.7 quake struck last night, the biggest earthquake yet at Bardarbunga.
On Sunday, Iceland lowered its warning code for possible volcanic disruption to the aviation industry to orange from red, after concluding that seismic activity had not led to a volcanic eruption under the glacier.
A red alert, the highest warning level, indicates an eruption is imminent or under way, with a significant emission of ash likely