Iceland has raised its alert level for the risk of a possible volcanic eruption to orange - the second most severe level.
The warning comes amid a number of earthquakes that have taken place since Saturday in Bardabunga, a subglacial stratovolcano located under Iceland's largest glacier.
Roads nearby have been closed over fears that an eruption could lead to massive flooding.
Scientists said there are two scenarios: one is an explosion outside the Vatnajokull glacier, leading to minor ash emissions and troubles locally.
The second possibility is an eruption occurring inside the glacier.
Seismologist Martin Hensch says the latter could lead to ash being sent high into the atmosphere.
In 2010 the ash from a volcanic eruption in Iceland caused major air traffic disruptions that stranded 10 million travellers around the world.