The risk of an eruption at Iceland's Bardarbunga volcano has increased, with signs of "ongoing magma movement", Iceland's meteorological office says.
The risk level to the aviation industry has been raised to orange, the second-highest level, the met office said.
Any eruption could potentially lead to flooding or an emission of gas, the office added in a statement.
The Eyjafjallajokull volcano erupted in 2010, producing an ash cloud that severely disrupted European airspace.
The Bardarbunga volcanic system is located under the north-west region of Iceland's Vatnajokull glacier.
"Intense seismic activity" began at the volcano on 16 August, and there was a strong earthquake in the region early on Monday, the met office said.
"This is the strongest earthquake measured in the region since 1996.
"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 met office said, adding that the situation was being monitored.
The aviation colour codes are used to indicate the level of risk a volcano poses to air travel.
An orange alert indicates that a volcano is showing "escalating unrest with increased potential of eruption", or is erupting without any major volcanic ash emissions.
A red alert means that an eruption is "imminent", or is under way with "significant emission of volcanic ash into the atmosphere".
The ash cloud produced by the 2010 Eyjafjallajokull eruption led to hundreds of thousands of passengers being affected by flight restrictions.