A new volcano erupted on the Reykjanes peninsula in southwestern Iceland late Thursday, spewing hot lava into the air in the sixth eruption to hit the region since December, authorities said.

Live video images showed orange lava bursting out of a long fissure, illuminating the billowing smoke rising up into the night sky.

"A volcanic eruption has begun. A fissure has opened east of Sylingarfell," the Icelandic Meteorological Office (IMO) said in a statement, adding that the eruption had started at 9:26 pm (2126 GMT) following a series of earthquakes.

The IMO initially estimated the length of the fissure at 1.4 kilometres (0.86 miles), adding in a later statement that it had extended to 3.9 kilometres in 40 minutes.

It said that there was still "considerable seismic activity" at the northern end of the fissure more than an hour after the start of the eruption.

A 4.0-magnitude earthquake was recorded at 10:37 pm, according to the IMO.

Dozens of people could be seen parked on the side of the main road from the capital Reykjavik to Iceland's Keflavik airport to watch the eruption, according to an AFP correspondent at the scene.

Iceland's national airport and air navigation service provider Isavia said in a statement that "flights to and from Iceland are operating normally despite the ongoing eruption."

This is the sixth eruption to hit the area since December, coming just two months after the end of a previous eruption that lasted more than three weeks.

The chief of police of the Sudurnes region, Ulfar Ludviksson, told Icelandic media that the evacuation of the nearby fishing village of Grindavik was going well.

He added that 22 or 23 houses in the village were currently occupied.

Most of Grindavik's 4,000 residents had evacuated  in November, prior to a December eruption, and while residents have since been allowed to return in between eruptions, only a few have opted to stay overnight.

- More favourable position -

Speaking to Icelandic public broadcaster RUV, Magnus Tumi Gudmundsson, professor of geophysics at the University of Iceland, said that unlike previous eruptions there was little activity on the fissure's southern end -- the direction of Grindavik.

"So if this continues as expected, there is no lava flowing near Grindavik. I think we have to consider it good news," Gudmundsson told the broadcaster.

The professor cautioned that the "night is young and we must continue to monitor this," but said that in general "it's a more favourable position than the last time."

The IMO has warned for weeks that another eruption was likely and said on Monday that "seismic activity" indicated a build-up of pressure to magma accumulation under Svartsengi, where a power plant that supplies electricity and water to around 30,000 people on the peninsula is located.

The Svartsengi plant was evacuated and has largely been run remotely since the first eruption in the region in December, and barriers have been built to protect it.

The Reykjanes peninsula had not experienced an eruption for eight centuries until March 2021.

Further eruptions occurred in August 2022 and in July and December 2023, leading volcanologists to warn that a new era of seismic activity had begun in the region.

Iceland is home to 33 active volcano systems, the highest number in Europe.

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