The UN says at least 340 people have now been killed while taking shelter at UN facilities in southern Gaza, as Israeli forces continue their offensive there.
Israeli forces are ramping up air and ground attacks on the city of Khan Younis in southern Gaza, which they regard as a key stronghold of the Palestinian Islamic group Hamas.
A compound of the UN Relief and Works Agency for Palestine Refugees in the Near East, or UNRWA, in Khan Younis came under tank assault on Wednesday, having been ablaze.
The head of UNRWA's Gaza office reported that nine people were killed and 75 injured in the attack.
A spokesperson of the US National Security Council said in a statement on Wednesday that "we are gravely concerned by reports" of strikes hitting the UNRWA facility.
The UN said on Wednesday that 1.2 million Gaza residents have sought safety at UN facilities in the enclave, and at least 340 of them have now been killed and more than 1,100 injured.
Qatar and Egypt are mediating talks for a pause in the fighting for the release of all remaining Hamas-held hostages.
The US government has disclosed that the US Middle East envoy, Brett McGurk, is in the Qatari capital Doha following his visit to the Egyptian capital Cairo to try to help mediate the pause.
But the prospect for the negotiations remains uncertain as discord is reported among negotiating parties.
Local media reported that Israeli Prime Minister Benjamin Netanyahu had expressed distrust of Qatar, suspecting that the Gulf state is supporting Hamas, and Qatar retorted to the reported remark.