COPENHAGEN (Reuters) -Icelandic authorities have completed the evacuation of 3,000 residents of a town in the southwest of the island over concerns of a volcanic eruption after a series of earthquakes and evidence of magma spreading underground.

The Icelandic Meteorological Office said on Saturday there was a "considerable" risk of an eruption on the Reykjanes peninsula because the size of the underground magma intrusion and the rate at which it was moving.

"I don't think it's long before an eruption, hours or a few days. The chance of an eruption has increased significantly," Thorvaldur Thordarson, professor of volcanology at the University of Iceland, told state broadcaster RUV.

Iceland's Civil Protection Agency overnight ordered a complete evacuation of Grindavik, a nearby fishing town, although it emphasized this was not an emergency evacuation.

The Reykjanes region has in recent years seen several eruptions in unpopulated areas, but the current outbreak is believed to pose an immediate risk to the town, authorities said.

On Thursday, increased seismic activity prompted the closure of the Blue Lagoon geothermal spa, one of the country's main tourist attractions.

Reykjanes is a volcanic and seismic hot spot southwest of the capital Reykjavik. In March 2021, lava fountains erupted spectacularly from a fissure in the ground measuring between 500 750 metres long in the region's Fagradalsfjall volcanic system.

Volcanic activity in the area continued for six months that year, prompting thousands of Icelanders and tourists to visit the scene. In August 2022, a three-week eruption happened in the same area, followed by another in July of this year.

The Fagradalsfjall system, which is around 6 km wide and 19 km long, had remained inactive for more than 6,000 years prior to the recent eruptions.

(Reporting by Jacob Gronholt-Pedersen and Louise Rasmussen in Copenhagen; Editing by David Holmes and Christina Fincher)