Iceland raised its alert over the nation's largest volcano to red on Saturday, banning all air traffic in the area, after detecting a small eruption.
A major explosion at the Bardarbunga volcano could signal a replay of the global travel chaos triggered when another peak blew four years ago, creating a massive ash cloud across Europe.
"There is an ongoing eruption beneath the glacial surface, probably a small eruption which has not been able to melt the ice cap," Met Office official Theodor Hervasson said.
The authorities earlier this week evacuated tourists and hikers from the area around Bardarbunga, which kicked into seismic action on Monday with the biggest earthquake registered since 1996.
However, police said there was no sign of a change at the surface of the erupting area and that the ice layer was between 150 and 400 meters (500 and 1,300 feet) thick.
"The eruption is considered a minor event at this point," Icelandic police said in a statement.
"Because of pressure from the glacier cap it is uncertain whether the eruption will stay sub-glacial or not."
The eruption of Eyjafjoell, a smaller volcano, in April 2010 caused travel mayhem, stranding more than eight million travellers in the biggest airspace shut down since World War II.
"There's nothing we can do if we get another big eruption like that of Eyjafjoell except to interrupt air traffic in the dangerous areas," Icelandic Civil Aviation Administration spokesman Fridthor Eydal was quoted as saying earlier this week.
"It's really the only thing we can do," he said.
The volcano is located in southeast Iceland under the country's largest glacier Vatnajoekull.
The area around it is uninhabited, with only trekking cabins and campsites used by tourists and hunters in the summer months.
Iceland's second-highest peak, Bardarbunga rises to more than 2,000 meters (6,500 feet), and caps the country's largest volcanic system.
On Monday, seismologists recorded an earthquake of 4.5 on the Richter scale in the area.
Scientists believe its explosion would be large enough to disrupt air traffic over northern Europe and the northern Atlantic, as well as causing major damage on the island nation from volcanic ash and glacial flooding.
In 2010, the Eyjafjoell volcano shot a massive plume of volcanic debris up to nine kilometers (six miles) into the sky, blowing ash across to mainland Europe.
And in 2011, Iceland's most active sub-glacial volcano Grimsvotn erupted, forcing Iceland to temporarily shut its airspace and sparking fears of a repeat of the Eyjafjoell flight chaos.