The UN Security Council on Friday called on Libya's warring parties to urgently de-escalate fighting and commit to a cease-fire, and it condemned a deadly attack on a detention centre for migrants near the capital Tripoli.

The UN's most powerful body also called on the parties to rapidly return to UN-mediated political talks.

The council press statement was the first approved by all 15 members since self-styled Libyan National Army leader, Field Marshal Khalifa Hifter, launched a military offensive on April 4 aimed at taking Tripoli, though it had previously issued weaker press "elements."

The statement was spurred by Tuesday night's airstrike on the detention centre in Tajoura, which the council said killed 53 people and injured over 130 others. The International Organization for Migration said Friday that six children were among those killed.

The Security Council held emergency closed consultations Wednesday afternoon, but diplomats said then that the United States needed additional time to consider the text of a statement condemning the attack.

A UN-supported, but the weak government controls Tripoli, overseeing the country's west, while a rival government aligned with Hifter controls much of the east and south. Each is backed by an array of militias and armed groups fighting over resources and territory.

The Tripoli-based government blamed Hifter's LNA and its foreign backers for the airstrike. The LNA, for its part, says it targeted a nearby militia position and denies striking the hangar where the migrants were being held. UN human rights chief Michelle Bachelet said the attack might amount to a war crime.

In Friday's statement, the council expressed "deep concern" at the worsening humanitarian situation in Libya and at conditions in detention centres it said, "are the responsibility of the Libyan government."

A civil war in Libya in 2011 toppled and later killed longtime dictator Moammar Gadhafi. In the chaos that followed, the oil-rich North African country was divided.

Hifter launched his offensive days before a national conference of all Libyan parties aimed at agreeing on elections for a new government was scheduled to begin — and despite his commitments to move toward elections.

He has received support from Egypt, France, the United Arab Emirates, Saudi Arabia and Russia and says he is determined to restore stability to Libya. The Tripoli-based Government of National Accord is backed by Turkey, Qatar and a number of militias, who observers say have put up stiff resistance against Hifter's forces.

The Security Council statement stressed that "lasting peace and stability in Libya will come only through a political solution."

Council members also stressed the importance of Libya's sovereignty, independence and territorial integrity "and called on all member states not to intervene in the conflict or take measures that exacerbate the conflict."

They reaffirmed support for the UN's Libya envoy, Ghassan Salame, and noted ongoing efforts in support of peace efforts by the African Union, the Arab League and others.

The European Union's Federica Mogherini, who serves as the organisation's foreign policy chief, also called for an immediate de-escalation of fighting, and a quick return to the UN-led mediation process.

In a statement, she urged UN member states "to fully respect their obligations to contribute to Libya's peace and stability, prevent destabilising arms shipments, safeguards Libya's oil resources" among other things.

The conflict is not only destabilising the region but "has increased the risk of terrorism and of the tragic loss of human lives, also at sea," she said.

"The EU stands ready to urgently step up collective efforts to address the situation and in particular to improve protection and assistance to migrants and refugees." She did not elaborate on what new efforts might be made.

Last month, UN Secretary-General Antonio Guterres strongly urged all countries to implement a UN arms embargo against Libya, saying illegal weapons transfers by land, sea and air are fueling the current fighting.

The Security Council also called for "full respect for the arms embargo."