UN Chief Antonio Guterres said he was "shocked by harrowing levels of death, injury and destruction" in north Gaza, where Israeli forces are carrying out attacks they say aim to prevent Hamas regrouping.
"The plight of Palestinian civilians trapped in North Gaza is unbearable," Guterres's spokesperson Stephane Dujarric said.
"The Secretary-General is shocked by the harrowing levels of death, injury and destruction in the north, with civilians trapped under rubble, the sick and wounded going without life-saving health care, and families lacking food and shelter."
The spokesperson said that according to Gaza's health ministry, hundreds of people have been killed in recent weeks and more than 60,000 others were forced to flee.
"Repeated efforts to deliver humanitarian supplies essential to survive - food, medicine and shelter - continue to be denied by the Israeli authorities, with few exceptions, putting countless lives in danger," Dujarric said.
"In the name of humanity, the Secretary-General reiterates his calls for an immediate ceasefire, the immediate and unconditional release of all hostages, and accountability for crimes under international law."