ZLITEN, Libya (AFP) - A suicide truck bombing on a police school in Libya's city of Zliten killed more than 50 people on Thursday, in the deadliest attack to hit the strife-torn country since its 2011 revolution.

A bomber detonated an explosives-laden truck used for carrying water at a police training centre in central Zliten at around 8.30am (2-30pm Singapore time), a local security source told AFP.

A witness in Zliten, a coastal city about 170km east of Tripoli, told AFP some 300 men, mainly coast guards, were inside the compound at the time.

Health ministry spokesman Ammar Mohammed Ammar said 50 to 55 people had been killed and at least 100 wounded and that victims were being treated in several hospitals.

Urgent calls were issued for blood donations.

Concrete buildings inside the compound were charred from the blast and their windows blown out, an AFP correspondent at the scene said. Nearby cars had been turned into black and twisted metal wrecks by the blast.

There was no immediate claim of responsibility for the attack but the Islamic State in Iraq and Syria (ISIS) group, which has been growing in power in Libya, has carried out many suicide bombings in the country.

Libya descended into chaos after the 2011 overthrow of longtime dictator Muammar Gaddafi and has had rival administrations since August 2014, when an Islamist-backed militia alliance overran Tripoli, forcing the government to take refuge in the east.

The internationally recognised government condemned the attack as a "cowardly terrorist act" and called for the lifting of an arms embargo it says has prevented authorities from tackling ISIS.

A deputy defence minister for the Tripoli-based government, Mohammad Bashir al-Naas, vowed to avenge the attack.

"The perpetrator is not known but he is a coward. He kills our sons from the shadows. We must avenge them and do everything possible to protect them," Naas told a news conference.

Hundreds of people braved the cold and high winds on Thursday afternoon to attend a prayer service for the dead at Zliten's stadium.

The United Nations is pressing Libya's rival sides to implement a power-sharing deal agreed last month on forming a unity government.

The UN envoy to Libya called for unity in the wake of the attack, saying implementing the political agreement was crucial.

"I condemn in the strongest terms today's deadly suicide attack in Zliten, call on all Libyans to urgently unite in fight against terrorism," UN envoy Martin Kobler wrote on Twitter.

EU policy chief Federica Mogherini also urged Libyans to back the unity deal.

"The people of Libya deserve peace and security and... they have a great opportunity to set aside their divisions and work together, united, against the terrorist threat facing their country," she said.

Italy, the former colonial power in Libya, offered its support in helping to bring stability.

"In the face of this terrorist threat, the first answer must be unity among Libyans," Foreign Minister Paolo Gentiloni said. "It is urgent that the recently signed political agreement be implemented."

World powers fear Libya could descend further into chaos and become an IS stronghold on Europe's doorstep.

In a report to the UN Security Council in November, International Criminal Court chief prosecutor Fatou Bensouda said that ISIS had been responsible for at least 27 car and suicide bombings in Libya in 2015.

The group claimed responsibility for suicide car bombings in the eastern town of Al-Qoba in February that killed at least 40 people.

In recent days, ISIS has launched a series of attacks on oil facilities in eastern Libya, pushing east from its coastal stronghold of Sirte.

Officials have warned of crippling consequences for the country if the jihadists manage to seize control of Libya's oil resources.

Calls have been growing for a possible foreign military intervention to bring stability to Libya and contain ISIS, which is reported to have at least 3,000 fighters in the country.

Mohamed Eljarh, a Libya analyst with the Atlantic Council, said the attack on Zliten was aimed as a show of strength and to highlight the vulnerability of security forces.

And despite the calls from the international community, he said it was unlikely the attack would boost unity efforts.

"This has not been the case in the past, even when ISIS was expanding and the scale of attacks was intensifying," Eljarh said.

"Despite ISIS' evident presence in Libya, various political groups are still consumed with their struggle for power and control."