(Adds scientist comment, changes to Rejkjavik/Stockholm dateline)
By Robert Robertsson and Simon Johnson
REYKJAVIK/STOCKHOLM, Aug 31 (Reuters) - A new eruption in Iceland's Bardarbunga volcano system spewed lava more than 50 metres in the air on Sunday prompting authorities to raise their warning of the risk of ash to aviation to the highest level on Sunday.
Iceland's largest volcanic system, which cuts a 190 km long and up to 25 km wide (118 miles by 15.5 miles) swathe across the North Atlantic island, has been hit by thousands of earthquakes over the last two weeks and scientists have been on high alert.
In 2010, an ash cloud from the Eyjafjallajokull volcano, in a different region of Iceland, closed much of Europe's air space for six days.
The latest eruption has not led to ash clouds, however.
"There is no ash, only lava," Eggert Magnusson at the National Crisis Coordination Centre said.
The current eruption began around 0600 GMT prompting the Icelandic Met Office to raise its aviation warning code to red from orange for the Bardarbunga area.
Red is the highest level on a five-colour scale and indicates that an eruption is imminent or under way, with a risk of spewing ash.
Iceland's aviation authorities have declared a danger area which reaches from the ground to 6,000 feet around the volcano.
The Department of Civil Protection and Emergency Management said the eruption is along a 1.5-kilometre front.
"The eruption is producing 50- to 60-metre high lava fountains," Armann Hoskuldsson, a geologist at the University of Iceland told Reuters.
"The lava flow from the fissure is about 10-20 times more than Friday morning."
Two days ago, a 600 metre-long fissure in a lava field north of the Vatnajokull glacier, which covers part of the Bardarbunga system, erupted.
That eruption only lasted for a few hours and was not in an area covered by ice and did not produce ash. The risk of an ash cloud is highest when there is a sub-glacial eruption as meltwater and magma mix to produce ash particles.
The new eruption is very close to Friday's and is not under the glacier.
"It is almost in the same location. The crack has only extended a little bit further to the north," Magnusson at the National Crisis Coordination Centre said.
Last week, scientists estimated around 400 million cubic meters of lava had flowed out from under the volcano in a long dyke. The eruption on Friday was at its tip. (Reporting by Robert Robertsson; Editing by Hugh Lawson)