An Icelandic town home to some 4,000 people near the capital Reykjavik could be heavily damaged by a volcano expected to erupt within hours or days, experts said on Saturday.

The town of Grindavik on the southwestern coast was evacuated in the early hours of Saturday after magma shifting under the Earth's crust caused hundreds of earthquakes in what was believed to be a precursor to an eruption.

"We are really concerned about all the houses and the infrastructure in the area," Vidir Reynisson, head of Iceland's Civil Protection and Emergency Management told AFP.

The town -- around 40 kilometres (25 miles) southwest of Reykjavik -- is located near the Svartsengi geothermal plant, the main supplier of electricity and water to 30,000 residents on the Reykjanes peninsula, as well as a freshwater reservoir.

Grindavik is also near the Blue Lagoon geothermal spa resort, a popular tourist destination which closed as a precaution earlier this week.

"The magma is now at a very shallow depth, so we're expecting an eruption within a couple of hours at the shortest, but at least within a couple of days," Reynisson said.

The most likely scenario would be a fissure opening in the ground near Grindavik.

"We have a fissure that's about 15 kilometres long, and anywhere on that fissure we can see that an eruption could happen," Reynisson said.

However he did not rule out the possibility of an eruption on the ocean floor, which would likely cause a large ash cloud.

"It's not the most likely scenario, but we can't rule it out because the end of the... fissure goes into the sea," he said.

The quakes and ground lift caused by the magma intrusion have already caused damage to roads and buildings in Grindavik and its surroundings.

A large crack also tore up the greens on the Grindavik golf course, an image widely shared on social media networks.

Iceland, which has 33 active volcanic systems, has declared a state of emergency and ordered the mandatory evacuation of Grindavik early Saturday.

Emergency shelters and help centres have opened in several nearby towns, but most Grindavik residents were staying with friends or relatives, media reported.

'Unprecedented' magma flow 

While the Icelandic Met Office (IMO) had for several days observed magma accumulating under the Earth's surface at a depth of about five kilometres, it said late Friday that the magma had begun rising vertically in a dyke.

"This magmatic dyke has been shallowing and the top depth has now been assessed to be 800 metres under the surface," IMO's volcanic hazards coordinator Sara Barsotti told AFP late Saturday.

She said experts were surprised by the amount of lava and the speed at which it was accumulating.

"What we are seeing now is an unprecedented event. We are talking about velocities for this process and volumes or inflow rates that are much higher than what we have seen on the peninsula so far."

Three eruptions have taken place on the Reykjanes peninsula in recent years near the Fagradalsfjall volcano: in March 2021, August 2022 and July 2023 -- all far from any infrastructure or populated areas.

The Earth's crust has been fractured "so much over the past three years" by those eruptions, "helping magmatic fluids in finding their path faster", Barsotti explained.

Prior to the March 2021 eruption, the Reykjanes peninsula had been dormant for eight centuries.

Volcanologists believe the new cycle of increased activity could last for several decades or centuries.

Situated in the North Atlantic, Iceland straddles the Mid-Atlantic Ridge, a crack in the ocean floor separating the Eurasian and North American tectonic plates.

A massive eruption in April 2010 at another Iceland volcano -- Eyjafjallajokull, in the south of the island -- forced the cancellation of some 100,000 flights, leaving more than 10 million travellers stranded.