TRIPOLI (AFP) - Gunmen claiming to be members of the Islamic State in Iraq and Syria (ISIS) militant group have seized control of a state-run radio station in Libya's coastal city of Sirte, residents said Friday.

Militant websites also posted pictures showing armed men sitting in front of microphones in a broadcasting studio and brandishing Kalashnikov assault rifles.

"They took Radio Sirte yesterday (Thursday). Since then they have been broadcasting (verses from) the Koran and speeches by (ISIS chief Abu Bakr) al-Baghdadi," a resident of the central city told AFP by telephone.

The resident said speeches of ISIS spokesman Abu Mohammed al-Adnani were also being played on the air.

A former local administration official said the gunmen have set up a headquarters in the centre of the city, and voiced concern that they could declare an Islamic emirate in Sirte.

"They could take advantage of the absence of any central government authority to transform the city into an Islamic emirate as they did in Derna," an eastern city held by extremist groups including ISIS.

The man, who declined to be named, said the gunmen apparently seized the radio "as a first step to communicate with the population".

"The situation in Sirte is very complex," he said, because many radical groups have a foothold there.

Since the 2011 uprising that toppled and killed dictator Moamer Kadhafi, Sirte, the former strongman's hometown has become a stronghold of extremist groups.

Sirte is a bastion of Ansar al-Sharia, an Islamist blacklisted by the United Nations and the United States.

And militias of the Islamist-backed Fajr Libya (Libya Dawn) coalition control some parts of Sirte, and launched attacks against key oil installations nearby in December.

On Friday, gunmen raided the Al-Bahi oil installation south of Sirte, causing heavy damage but no casualties before being repelled, guards at the privately run site said.

The security spokesman for Libya's oil facilities, Ali al-Hassi, pointed a finger of blame at ISIS, whose militants attacked the nearby Al-Mabrouk field on Feb 3.

Powerful militias, backing rival governments and parliaments, have been battling for control of key cities and the country's oil riches since the uprising four years ago.

Taking advantage of the chaos, ISIS has claimed a string of deadly attacks. The latest targeted a luxury hotel in Tripoli on Jan 27, when nine people were killed, five of them foreigners.