Iceland on Tuesday reported unusual seismic activity at Mount Hekla, one of its most active volcanos, and raised its alert level but said no eruption was imminent.
The Icelandic Met Office said it had raised its aviation warning level a notch Tuesday for the first time in two years following "elevated unrest above a known background level during the last weeks."
The office told AFP there was no smoke rising from the mountain, but dark clouds seen from the ground could lead people to think there had been an eruption.
The alert level for the aviation industry was raised from "green" to "yellow", which it described as a precautionary measure due to the volcano's "increased earthquake activity."
Emergency services said in a separate statement that they had declared "an uncertainty phase", the lowest level of warning issued for events "that may threaten public health and safety, or that an environment or inhabited area may be threatened."
The eruption of Iceland's Eyjafjoell volcano in April 2010 stranded more than eight million travellers as volcanic ash spread across Europe.