The UN Security Council has failed to agree a joint statement denouncing an air strike on a migrant detention centre in Libya where 44 people were killed and more than 130 were seriously injured.
Reports citing diplomats in New York indicate that the United States was not prepared to support the text of a proposed statement without approval from Washington.
Libya's UN-backed-government and rebel militias have blamed each other for the attack.
During a two-hour closed-door meeting, Britain circulated a statement that would have condemned the deadly air strike blamed on commander Khalifa Haftar's forces, called for a ceasefire and a return to political talks.
But US diplomats told the meeting that they required a green light from Washington to approve the text and the talks ended without US approval, sources told AFP.
At least 44 people were killed when the air strike hit the Tajoura detention centre late on Tuesday and more than 130 were severely wounded, according to the UN.
Peru's Ambassador Gustavo Meza-Cuadro, whose country holds the council presidency, told reporters that discussions were continuing on the press statement but council diplomats said they did not expect a response.
Mr Haftar, who holds sway in eastern Libya, launched an offensive in April to seize Tripoli, seat of a UN-recognised government that is backed by various militias.
World powers have been divided on how to respond to Mr Haftar's military campaign, with the US and Russia refusing to back UN calls for a ceasefire.
The proposed statement would have called on warring sides to "immediately de-escalate the situation and commit to a ceasefire".
The government of national accord, based in Tripoli, said Mr Haftar's forces were responsible for the strike on the detention center.
A US State Department statement released in Washington earlier condemned the "abhorrent" air strike but did not call for a ceasefire.
UN Secretary-General Antonio Guterres earlier said he was outraged and called for an independent investigation into the deadly strike.
He recalled that the United Nations had shared the coordinates of the detention center with the warring sides to ensure that civilians sheltering there were safe.