A volcano erupted about 30km from the Icelandic capital of Reykjavik on Monday.
It is the volcano's third eruption in two years, with local media footage showing a huge cloud of smoke rising from the ground and a substantial flow of lava at the site.
The smoke can be seen from the road connecting the capital to the international airport, with cars pulled over and people taking pictures, AFP reported.
"The eruption is taking place in a small depression just north of Litli Hrutur ['Little Ram' in Icelandic] from which smoke is escaping in a north-westerly direction," the meteorological office said.
Thorvaldur Thordarson, professor in volcanology at the University of Iceland, told AFP: "There are three fissures with lava basically running in all directions.
Prof Thordarson said the fissures were in total about 200 to 300 metres long.
"It is a low-intensity, effusive eruption," he said.
This means "it's not causing widespread threats due to explosive activity" but "if the eruption continues for long enough it could be a threat to infrastructure".
Thousands of small earthquakes were recorded in the area in the week leading up to the eruption, indicating that the magma below the ground was moving and an eruption was imminent.
Icelandic volcano erupts - in pictures
The Icelandic authorities advised against going to the site, which is in difficult terrain without road connections, before they have assessed the situation.
The magma broke through the ground about 4.40pm, just a few kilometres from two previous eruptions in the past two years.
The first was on March 19, 2021, in the Geldingadalur valley and lasted six months, while the second occurred on August 3, 2022, in the Meradalir valley, lasting three weeks.
Before the 2021 eruption, the region had remained dormant for eight centuries, but volcanologists believe the new cycle of increased activity could last several years.
The effusive eruptions that have occurred in this area so far have not been very dangerous, and they have not had any effect on air traffic.
The 2021 and 2022 eruptions attracted hundreds of thousands of visitors hoping to catch a rare glimpse of an active volcano.
Prof Thordarson said the eruption could last from "a few days" to more than half a year like in 2021, or even longer.
Iceland has 33 volcanic systems considered active, the highest number in Europe. It has an eruption every five years on average.
The North Atlantic island straddles the Mid-Atlantic Ridge, a crack in the ocean floor separating the Eurasian and North American tectonic plates.
In April 2010, about 100,000 flights were cancelled, leaving more than 10 million travellers stranded, after the massive eruption of the Eyjafjallajokull volcano.
Other volcanoes, such as Askja in the uninhabited highlands of central Iceland, have recently shown signs of activity.
One of the country's most dangerous volcanoes is Katla, near the south coast. It last erupted in 1918, with an unusually long pause suggesting an imminent reawakening.
The 1783 eruption of the Laki volcanic fissure in the south of the island is considered by some experts to be the most devastating in Iceland's history, causing its biggest environmental and socio-economic disaster.
Between 50 to 80 per cent of Iceland's livestock was killed, leading to a famine that left a quarter of the country's population dead.
The meteorological impact of the eruption also had repercussions for years, with some experts suggesting it may have played a part in starting the French Revolution.
