The UN's secretary-general says "aid has dried up [and] the floodgates of horror have re-opened" in the Gaza Strip, where Israel has blocked the entry of all goods and resumed the war against Hamas.
"Gaza is a killing field, and civilians are in an endless death loop," António Guterres said on Tuesday.
His comments come after the heads of six UN agencies appealed to world leaders to act urgently to ensure food and supplies reached Palestinians there.
Israel's foreign ministry insisted there was enough food in Gaza and accused Guterres of "spreading slander against Israel".
Israel blockaded Gaza on 2 March, after the first stage of a ceasefire expired. Hamas refused to extend that part of the truce, accusing Israel of reneging on its commitments.
Israel then renewed its aerial bombardment and ground offensive on 18 March and these have since killed 1,449 Palestinians, according to the Hamas-run health ministry in Gaza. The Israeli military insists it does not target civilians.