(Adds Met Office, Crisis Coordination centre comment)
By Robert Robertsson and Simon Johnson
STOCKHOLM, Aug 31 (Reuters) - A small new fissure eruption in an ice-free area of Iceland's Bardarbunga volcano system prompted 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 eruption is a very calm lava eruption and can hardly been seen on seismometers," the Icelandic Meteorological Office said in a statement.
"There is no ash, only lava," Eggert Magnusson at the National Crisis Coordination Centre said.
The eruption began around 0600 GMT prompting the Icelandic Met Office to raise the aviation warning code to red for the Bardarbunga/Holuhraun area, the Department of Civil Protection and Emergency Management said in a statement.
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.
On Friday, 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. 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.
"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.
Scientists, however, said the new eruption was more intense than that two days ago.
"This is a little bit larger fissure eruption than on Friday," Armann Hoskuldsson, a geologist at the University of Iceland who is working in the area, told Reuters.
"There is more lava and more rifts in the ice cap. The rifts are approximately 1 km further to the north than after the fissure eruption on Friday."
He said the lava flow now is at least 10 times larger than in the fissure eruption on Friday.
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)