The Icelandic Met Office (IMO) raised the aviation colour code from orange to red Saturday as a small sub-glacial volcanic eruption started near Bardarbunga volcan.
The IMO said the small lava eruption, though not visible, was detected under Dyngjujokull glacier in the northern part of Vatnajokull glacier. An earthquake, estimated at magnitude of 4.5, was also detected by the office in the afternoon, Xinhua reported.
The aviation colour code for the Bardarbunga volcano has been changed from orange to the highest red, resulting in the air space above the eruption site being closed.
The national commissioner of the Icelandic Police (NCIP) has raised the alert phase to emergency phase accordingly following the eruption.
The NCIP said in a statement the eruption was considered as a minor event at this point, and it was uncertain whether the eruption would stay sub-glacial or not.
A coast guard's aircraft, carrying observations with scientists from IMO, the Institute of Earth Sciences, and people from the civil protection, were monitoring the volcano and saw no sign of change on the surface.
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The NCIP said that measures taken at this stage were based on a small event. The Jokulsargljufur canyon has been closed, and they were evacuating tourists in that area and around Dettifoss waterfall.
Bardarbunga volcano, located under the 800-metre-thick Vatnajokull glacier, is Iceland's second highest mountain at 2,000 metres above sea level and also one of the most dangerous volcanoes.
Since the onset of the earthquake swarm at Bardarbunga last Saturday morning, thousands of earthquakes have been detected around that area.