Iceland volcano: Bardarbunga hit by 5.7 earthquake
Iceland's Bardarbunga volcano was hit by a magnitude 5.7 earthquake on Tuesday morning, the largest since tremors began in the area last week.
The country's Met Office said despite the shock - the fourth magnitude five quake in 48 hours - there is still no sign of a volcanic eruption.
On Sunday, Iceland lowered the aviation risk to its second highest level.
Iceland's Eyjafjallajokull volcano erupted in 2010, producing ash that disrupted air travel across Europe.
Bardarbunga is in a different range to Eyjafjallajokull, but the intense seismic activity has raised fears that an eruption could cause similar travel chaos.No surface movement
"There was one event during the night. It was a magnitude 5.7 [earthquake], the largest in this series," Palni Erlendsson, a geologist at Iceland's Met Office told Reuters news agency.
The activity was still deep below ground, Mr Erlendsson said, adding there had been no signs of movement close to the surface.
"We still can't say whether [the activity] will cease, continue like this for a while or erupt. It's impossible to say," he said.
The magnitude five earthquakes are the strongest to hit the Bardarbunga region since 1996.
The Met Office said the latest large tremor occurred at 01:26 GMT to the north of Bardarbunga, 6km (3.7 miles) below the surface, near the rim of Dyngjujokull, another nearby volcano.
Bardarbunga and Dyngjujokull are part of a large volcano system hidden beneath the 500m-thick (1,600ft) Vatnajokull ice cap in central Iceland.
The region, more than 300km (190 miles) from the capital Reykjavik, has no permanent residents but sits within a national park popular with tourists. Several hundred people have been evacuated from the area.
Authorities have previously warned that any eruption could result in flooding north of the glacier.
But experts said the activity was migrating north, with the tip of the movement already 10km (6 miles) outside the glacier.
Scientists believe the earthquakes are a result of magma flowing out from beneath the volcano, causing a change in pressure.
This movement could stop, reducing the seismic activity, or the magma could reach the surface and lead to an eruption.
On Sunday, Iceland lowered its level of alert to the aviation industry from red, warning of an imminent eruption, to its second-highest level, orange.
Airspace over the site has been closed, but all Icelandic airports currently remain open, authorities say.
The Eyjafjallajokull eruption in April 2010 caused the largest closure of European airspace since World War Two, with losses estimated at between 1.5bn and 2.5bn euros (£1.3bn-2.2bn).