UN chief Antonio Guterres denounced the "horrendous" attack on a detention center in Tripoli and demanded an independent investigation. A divided Security Council failed to condemn the strike.
The council president said a statement was due: "We will have a press statement, but we are still discussing it," Peruvian Ambassador Gustavo Meza-Cuadra said.
The airstrike hit the Tajoura detention center in the suburbs of Tripoli on Wednesday. Libya's Tripoli-based government, which is backed by the UN, has blamed the attack on rival Libyan National Army forces loyal to rogue general Khalifa Haftar.
Read more: Could Libya be Russia's new Syria?
During a two-hour closed-door meeting of the Security Council, Britain circulated a statement that condemned the airstrike and called for a ceasefire.
According to diplomatic sources, the US prevented the 15-member Security Council from issuing a statement although it was unclear why.
A US State Department statement released in Washington earlier had condemned the "abhorrent" airstrike but did not call for a truce.
Nearby military target
The United Nations and the European Union have both urged an investigation into the attack.
Haftar's forces said they were targeting a nearby military site, rather than the detention center. Other countries suspected of alignment with his command include Russia, as well as US allies Egypt and the United Arab Emirates.
Read more: Khalifa Haftar: Libya's military strongman
As night fell, the International Organization for Migration said an estimated 250 migrants remained at Tajoura.
Charlie Yaxley, a spokesman for the UNHCR refugee agency, said the detention center's proximity to the military depot "made it a target for the airstrikes."
"Coordinates of this detention center were well-known to both sides of the conflict," Yaxley said, adding that the UNHCR was sending medical teams to the site.
rc/se (AFP, AP, Reuters, dpa)