Geologists say an increased swarm of earthquakes around the Bardarbunga volcano tell them there is "no doubt" lava is rising, increasing the risk of an eruption, Britain's Daily Telegraph reported this week.
Pall Einarsson, a professor of geophysics at the University of Iceland, says the signs of increased activity provide "good reason to worry."
The sustained earthquake tremors in the remote volcano range are the strongest recorded in recent times, he said.
"This is the most active area of the country if we look at the whole country together," he said. "There is no doubt that lava there is slowly growing, and the seismicity of the last few days is a sign of it."
The last recorded eruption of Bardarbunga was in 1910.
Bardarbunga is much larger than the Eyjafjallajokull volcano, which shut down air travel across most of Europe last year when its ash cloud drifted across the continent.