Iceland has raised its warning level to aircraft after two earthquakes hit one of the country's biggest volcanoes.

The Icelandic Met Office said the quakes, one of which had a magnitude of 3.7, shook the Katla Volcano in the south of the country.

They are the strongest tremors since quakes raised concerns over a possible eruption of Katla last month.

Meteorologists said an "intense seismic swarm" had been recorded since Thursday morning.

Earthquake swarms occur when sequences of quakes strike in a relatively short period of time.

The Icelandic Met Office said: "Due to the unusually high level of unrest at the Katla volcano, we raise the aviation colour code from green to yellow."

Yellow is the second lowest of the four-colour warning scale.

The Katla Volcano has not had a major eruption since the beginning of the last century, but scientists think a large eruption is overdue.

However, they admit it still might be decades before it happens.

Last month, the volcano was rocked by two quakes measuring 4.5 and 4.6 in magnitude, a level not recorded since 2011.

In April 2010, the nearby Eyjafjallajokull Volcano erupted, causing a major part of Europe's airspace to be closed for several days.