At least 47 people have been killed and 118 injured in an apparent suicide bomb attack at a police training centre in Western Libya.

The attack, which has been labelled the worst since the fall of Muammar Gaddafi, took place as around 400 hundred police and coastguard cadets gathered for their morning meeting in the town of Zliten.

The town lies between the capital Tripoli and the port of Misrata.

"It was horrific, the explosion was so loud it was heard from miles away," Mayor Miftah Hamadi told Reuters by telephone. "All the victims were young, and all about to start their lives."

According to Reuters, the charred remains of a vehicle lay strewn across the ground near the police academy building and parked cars were mangled by the force of the blast.

Witnesses said residents ferried victims to Misrata hospitals in ambulances and cars, many with shrapnel wounds and some bodies too damaged to be identified.

Medical sources initially said 65 people had been killed, including some civilians. But Fozi Awnais, head of the crisis committee for the health ministry in Tripoli, said later that 47 people had died and 118 more were wounded.

The attack was not immediately claimed by a group, however suicide blasts and car bombings have increased in Libya as Islamist militants have taken advantage of the North African country's chaos to expand their presence.

Thursday's bombing follows a number of truck bombings in Misrata and further east claimed by ISIS.

Late on Thursday ISIS claimed another car bombing that killed at least seven in the oil port of Ras Lanuf. The port and a nearby terminal at Es Sider came under attack from militants earlier this week, in their most concerted effort to date against Libya's oil infrastructure.

Since a NATO-backed revolt ousted Gaddafi, Libya has slipped deeper into turmoil with two rival governments and a range of armed factions locked in a struggle for control of the OPEC state and its oil wealth.

In the chaos, ISIS militants have grown in strength, taking over the city of Sirte and launching attacks on oilfields.

On Monday, ISIS fighters launched an attack on Ras Lanuf and Es Sider, Libya's biggest oil export terminals, which are situated between Sirte and the eastern city of Benghazi.

Clashes over three days had already had already left 11 guards dead and set seven oil storage tanks on fire before Thursday evening's car bombing.

The ports have been closed since December 2014, and Libya's oil output has plunged to less than one quarter of a 2011 high of 1.6 million barrels per day.

The Zliten blast was the worst since an attack in February last year when three car bombs hit the eastern city of Qubbah, killing 40 people in what officials described as a revenge attack for Egyptian air strikes on Islamist militant targets.

Thursday's bombing has called into question Britain's current plans to deploy up to 1,000 troops into the country early this year, according to the Guardian. The police centre which was targeted on Thursday was one of the major training bases where British troops were planning to be based.

Western powers are pushing Libya's factions to back a UN-brokered national unity government to join forces against ISIS militants, but the agreement faces major resistance from several factions on the ground.

For more than a year, an armed faction called Libya Dawn has controlled Tripoli, setting up its own self-declared government, reinstating the former parliament and forcing the recognised government to operate in the east of the country.

Western officials say forming a united government would be the first step in Libya seeking international help to fight against ISIS, including training for a new army and possible air strikes against militant targets.

Additional reporting by Reuters.