At least 30 people have been killed and 33 others wounded in an air strike at a military school in the Libyan capital, Tripoli, officials have said.
The UN-backed government blamed rebel forces loyal to Gen Khalifa Haftar for the attack. The rebels denied involvement.
Footage from the scene showed bodies scattered across the ground.
Gen Haftar's troops launched an offensive in April to try and take control of Tripoli.
The foreign ministry called for an emergency session of the UN Security Council to discuss the air strike, and said Gen Haftar should be investigated by the International Criminal Court for alleged war crimes.
Libya has been torn by violence since long-time ruler Muammar Gaddafi was deposed and killed in 2011 by Nato-backed forces.
It has two rival administrations, based in Tripoli and the eastern city of Tobruk.
The conflict has increasingly drawn in foreign states, with Turkey's parliament voting last week to deploy troops to support the UN-backed government in Tripoli.
Gen Haftar is allied with the Tobruk administration, and is the main military commander fighting the Un-backed government.
He has the support of the United Arab Emirates, Egypt, Jordan and Russia.
Libya is a major oil producer, and is used as a transit point by migrants trying to reach Europe.