Guterres recalled that the United Nations had shared the coordinates of the Tajoura detention center east of Tripoli with the warring sides to ensure that civilians sheltering there were safe.

The UN chief "condemns this horrendous incident in the strongest terms," said a statement from his spokesman.

He "calls for an independent investigation of the circumstances of this incident, to ensure that the perpetrators are brought to justice, noting that the United Nations had provided exact coordinates of the detention center to the parties."

At least 44 people were killed when the strike hit the center late Tuesday and more than 130 were severely wounded, the UN said.

The UN Security Council is due to meet behind closed doors later Wednesday to discuss Libya, a divisive issue among world powers that have backed rival sides in the conflict.

Tensions have soared since forces loyal to commander Khalifa Haftar, who holds sway in the east of the country, launched an offensive in April to seize Tripoli, held by a UN-recognized government and various militias.

The Tripoli government has blamed Haftar for the attack on the detention center.

Guterres renewed his call for an immediate ceasefire and a return to dialogue.

UN envoy Ghassan Salame separately also called for an independent investigation and warned that the attack may constitute a war crime.