It is feared that meltwater flooding from the Grimsvotn glacial lake in Iceland could signal that the volcano underneath is about to erupt.
Iceland Civil Protection Department spokeswoman Gudrun Johannesdottir said that water now pouring from Iceland's biggest glacier, Vatnajokull, could be a sign of fresh geological activity.
Eyjafjallajokull, the volcano that caused severe disruption to air travel earlier this year, is about 100km southeast of Vatnajokull.
In April, clouds of ash grounded flights across Europe for a week, causing billions of euro in losses for airlines and other industries.
Gudrun Johannesdottir said that 'sometimes an eruption occurs when a glacial outburst flood starts, but not every time. But we are monitoring the situation closely.'
The last eruption at Grimsvotn was in 2004 and caused short-term disruptions to airline traffic into Iceland.