Iceland's largest volcanic system has been struck by a magnitude 5.5 earthquake overnight, one the largest tremors since seismic activity started, according to the country's Meteorological Office.

The earthquake shook the northern rim of the Bardarbunga caldera shortly after 3am local time.

According to the Met Office, the tremors appear to be becoming more frequent. In the past few days, volcanic activity has increased dramatically, with the lava now covering an area of more than four square kilometres.

Three other tremors of magnitudes higher than 4.0 have occurred in the past 24 hours, the Iceland Review reports.

Overall, around 500 earthquakes were recorded around Bardarbunga yesterday, with most minor tremors by the Dyngjujökull glacier.

Volcanologists have expressed concern that the fissure, which appeared in a lava field near the Vatnajokull glacier and expands to the north, may spread south.

If it reaches below the glacier, along with the magnitude 5 tremors, it could case a violent eruption. The melting of the ice could cause flooding and major explosions.

No ash has so far been observed by scientists monitoring the fissure, which is situated around 28 miles from the main Bardarbunga volcano.

The volcano itself lies under the Vatnajokull glacier, the largest ice cap by area in Europe.

After briefly raising the aviation warning to red, the highest level which indicates an eruption is imminent or underway with a significant emission of ash likely, the warning was lowered to orange.

On Sunday 31 August, after lava fountains reached up to 50m in the air, the Met Office upped the flight warning level to red.

"The fissure eruption is continuing at a stable level," the Met Office said in a statement on Monday. "No explosive activity is observed, the eruption remains an effusive lava eruption."

"Visual observation confirms it is calm, but continuous," officials said at the weekend.