No group immediately claimed the attack in the western town of Zliten, but 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.

Late on Thursday the Islamic State militant group 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.

In Zliten, Mayor Miftah Hamadi said the truck bomb detonated as around 400 recruits were gathering in the early morning at the police center. Zliten lies between the capital Tripoli and the port of Misrata.

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