A swarm of earthquakes under a volcano in Iceland has prompted the country to increase the alert level of a potential volcanic eruption to orange, its second-highest level.

Thousands of small but intense earthquakes have been recorded in the last few days in the region of Bardarbunga, a stratovolcano buried under the country's biggest glacier, the Icelandic Meteorological Office reported.

One tremor on Monday was the strongest felt in the region since 1996.

Roads in the area have been ordered closed, out of fear an eruption might cause massive flooding from melting ice and snow on the overlying Vatnajokull glacier.

There is also fear of a massive outpouring of ash into the upper atmosphere like that created by the 2010 eruption of Eyjafjallajokul, when ash drifting into European air space caused the cancellation of almost 100,000 airline flights.

Those cancellations had a ripple effect, stranding more than 10 million travelers across the globe.

Instruments around Bardarbunga show magma is moving under the volcano, although two clusters of the molten rock detected are distant from the center of the main volcano outlet, suggesting an eruption might cause local melting and flooding but not the explosive ash-producing outburst like Eyjafjallajokul, seismologists say.

Nevertheless, the activity has caused an orange aviation alert, issued because the "volcano shows heightened or escalating unrest with increased potential of eruption," the meteorological office said, noting the "ongoing magma movement."

Experts have been keeping an eye on Bardarbunga for a long time.

"We have known for some time that Bárðarbunga was going to do something -- we just didn't know what," says volcanologist Dave McGarnie of Britain's Open University. "There is no way to predict when the eruption may happen, but we should get a few hours' notice."

Bardarbunga is part of Iceland's largest volcanic system, with fissures and vents stretching for more than a hundred miles.

It is one of Iceland's less-studied volcanoes because of its remote location far from any Icelandic cities and its history of infrequent eruptions, about one every 250 to 500 years.

Many of its eruption have not broken through its covering glaciers, further complicating studies of the volcano and predictions of possible future eruptions.

The authorities in Iceland say they are continuously monitoring the current activity.

"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 meteorological office said.