Iceland has declared a state of emergency after a series of earthquakes raised fears of a volcanic eruption.
Authorities have ordered thousands living in the southwestern town of Grindavík to evacuate as a precaution.
The Icelandic Met Office (IMO) says it is concerned large amounts of magma - molten rock- is spreading underground and could surface there.
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.
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".
In a statement 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.
In an statement on Friday, the IMO said "significant changes have occurred in the seismic activity", with tremors moving towards Grindavík over the course of the day.
It added that magma has likely extended beneath the town and it was "not possible to determine exactly" whether or where it could emerge.
"The amount of magma involved is significantly more than what was observed in the largest magma intrusions associated with the eruptions at Fagradalsfjall," the IMO said.
Iceland is one of the most geographically 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.
