Fighting erupted in the city of Derna after a leader in the Islamist umbrella group Majlis al-Shura was killed, apparently for refusing to swear allegiance to ISIS chief Abu Bakr al-Baghdadi, according to residents, Reuters reported.

Meanwhile, another Islamist force that later declared jihad against its hardline rival, residents and medical sources said on Wednesday.

ISIS and other Islamist groups have thrived in Libya since two governments began fighting for control of the oil producer, leaving the country in a security vacuum four years after the ousting of Muammar Qaddafi.

The rapid rise in Libya of ISIS, which first emerged in Syria and Iraq, is worrying Western powers who fear the militant group will forge a base just across the Mediterranean from mainland Europe.

The fighting in Derna lasted until Wednesday, by which time Majlis leader Salem Derbi and around 18 ISIS fighters had been killed, medical sources said.

Majlis al-Shura later declared jihad, or holy war, on ISIS. The battle appears to have been the largest yet between ISIS and rivals in Derna.

“We …have warned those ISIS militants to stop their criminal acts but they were committing even more crimes,” the statement said.

ISIS has expanded its territory to include the whole of the central city of Sirte, and Harwa to the east. The group has carried out attacks on oilfields and embassies and also claimed the killing of dozens of Egyptian and Ethiopian Christians.

Angry supporters of the Majlis al-Shura stormed several buildings held by ISIS during the fighting, while shops closed and frightened residents stayed indoors on Wednesday, according to residents.

Libya is in chaos as two governments controlling limited territory are fighting each other, with ISIS attacking forces of both administrations.

The official government has been based in the east since losing the capital in August to rival group Libya Dawn, which set up its own administration with some Islamist links.