Islamic State militants on Wednesday, launched a sudden attack in Libya’s western Sabrata city, killing several policemen, before seizing the police headquarters and hanging their black flags on top of the building, local sources said.

On Tuesday, unofficial local armed forces stormed several sites in the south of the city in search of Islamic State militants-this caused a security gap in central Sabrata, which allowed the militants to enter the area, Libyan Cloud News Agency reported.

The local military council chief, Taher al-Gharabi, told newsmen that the attack was believed to be carried out by sleeper cells in central Sabrata.

Al-Gharabi said that between 150 to 200 fighters took control of several buildings , including the police headquarters adding that six policemen and five extremists were killed in the process.

Most of the extremists are believed to be Tunisians, while some are Libyans, al-Gharabi said.

Sabrata Mayor , Hussien Dawadi was quoted by reports as saying that Libyan forces began arriving from neighbouring towns to Sabrata to support local troops that had besieged the town in order to prevent Islamic State militants from fleeing the area.

It was not immediately clear whether the police headquarters was freed or whether the extremists managed to flee the area.

There is growing concern and evidence that Islamic State is boosting its presence in Libya, which was thrown into chaos after the NATO-backed ouster of longtime dictator Mammaur Gaddafi in 2011, as the extremist group comes under pressure and cedes ground in both Syria and Iraq.

Islamic State already controls a long stretch of coastline around the city of Sirte, about 500 kilometres east of Tripoli.

In mid February, the US launched an airstrike against Islamic State extremists in the city, killing scores.

The airstrike targeted Noureddine Chouchane, a Tunisian operative linked to two major attacks in Tunisia last year.

Sabrata, located 80 kilometres away from the Tunisian border, is seen as a hotbed for foreign nationals who have joined Islamic State.