A hospital in Gaza reported that at least 37 Palestinians were killed by an Israeli attack on a UN-operated school on Thursday.
The raid came after US, Qatari and Egyptian mediators resumed talks aimed at securing a truce and hostage-prisoner swap in the eight-month war that has devastated the Palestinian territory.
The United States has urged Israel to provide complete transparency regarding the strike.
"The government of Israel has said that they are going to release more information about this strike, including the names of those who died in it. We expect them to be fully transparent in making that information public," State Department spokesman Matthew Miller told reporters.
UN chief Antonio Guterres called the strike "just another horrific example of the price that civilians are paying".
"There will need to be accountability for everything that has happened in Gaza," his spokesman Stephane Dujarric said.
EU foreign policy chief Josep Borrell called for the strike to be "independently investigated".
The Al-Aqsa Martyrs hospital in Deir al-Balah, near Nuseirat, said it had received the bodies of at least "37 martyrs" from the strike.
