The chance of a volcanic eruption in Iceland is rising, posing a threat to a now-evacuated town, experts say.

Iceland has declared a state of emergency after a series of earthquakes.

Authorities have ordered thousands of people living in the southwestern town of Grindavík to leave as a precaution.

The Icelandic Meteorological Office (IMO) said there was a considerable risk of an eruption.

The probability of an eruption on or just off the Reykjanes peninsula has increased since the morning, IMO says.

An eruption could start at any time in the next few days, according to the statement.

Thor Thordason, professor of volcanology at the University of Iceland, said a 15km-long (nine mile) river of magma running under the peninsula was still active.

"That's why we're talking about an imminent eruption unfortunately. The most likely eruption side appears to be within the boundary of the town of Grindavík," he told the BBC.

Thousands of tremors have been recorded around the nearby Fagradalsfjall volcano in recent weeks.

They have been concentrated in Iceland's Reykjanes Peninsula, which had remained dormant to volcanic activity for 800 years before a 2021 eruption.

In a statement on Saturday the agency said a tunnel of magma, or molten rock, that extends northeast across Grindavík and some 10km further inland, was estimated at a depth of less than 800 metres, compared with 1,500 metres earlier in the day.

On Thursday, the increased seismic activity in the area prompted the closure of the nearby Blue Lagoon landmark.

More than 20,000 tremors have been recorded in southwest Iceland since late October.

Iceland's Civil Protection Agency said the decision to evacuate came after the IMO could not rule out a "magma tunnel that is currently forming could reach Grindavík".

And on Friday, the agency said people must leave the town, but also emphasised it was not an "emergency evacuation" - calling on them to "remain calm, because we have a good amount of time to react".

"There is no immediate danger imminent, the evacuation is primarily preventive with the safety of all Grindavík residents as the principal aim," it added.

All roads into the town of around 4,000 people are closed other than for emergencies, to ensure traffic can get in and out.

Alda Sigmundsdottir, a journalist in Reykjavik, said that people were going back into the town "to get their absolute bare necessities" and pets.

"We are just currently waiting for the eruption to start," she told the BBC's Newshour.

Iceland is one of the most geologically active regions in the world, with around 30 active volcanic sites.

Volcanic eruptions occur when magma, which is lighter than the solid rock around it, rises to the earth's surface from deep below it.

In July, Litli-Hrutur, or Little Ram, erupted in the Fagradalsfjall area, drawing tourists to the site of the "world's newest baby volcano".

The site was dormant for eight centuries until eruptions in 2021, 2022 and 2023.