Iceland volcano Bardarbunga rumbling threatens air travel

The world's airline industry is on alert after Icelandic authorities have raised the alarm over a sudden flurryof small earthquakes that could cause one of the country's largest volcanoes to erupt.

The Icelandic Meteorological Office warned Tuesdayit has detected more than 3,000minor earthquakes since Saturday along a fault line in the country's centre.

Much of the activity is clustered around Bardarbunga a subglacial stratovolcano under Iceland's largest glacier. "Several of these events were larger than magnitude 3," the office warns, with one touching 4.5 on the Richter scale the strongest in that region since 1996 although none has been that strong in the past 24 hours.

Although seismic activity in Iceland is routine, the sudden increase in number and intensity has volcanologists worried, because the last time similar activity was seen was in April 2010, whenEyjafjallajokullerupted and the plume of ash and smoke grounded transatlantic air travel for the better part of a month.

Seismologists say magma is movinghorizontally, not yet vertically, and "no signs of migration towards the surface or any other signs of imminent or ongoing volcanic activity have been detected so far."

However, the aviation alert level has been raised to the second-most severe level as a precaution.

"Presently there are no signs of eruption, but it cannot be excluded that the current activity will result in an explosive subglacial eruption, leading to an outburst flood and ash emission," the office said.