World efforts to end two weeks of deadly violence in and around Gaza stepped up a gear on Monday as the US top diplomat and the UN chief both headed to Cairo.
US Secretary of State John Kerry was flying to the region after President Barack Obama urged an "immediate ceasefire", echoing a call by the UN Security Council.
UN chief Ban Ki-moon was already in the region on a whistlestop tour to build support for a truce and was to meet the ruler of Kuwait, current chair of the Arab League, before heading to the Egyptian capital.
The new momentum for a ceasefire came as the Palestinian Arab death toll in Gaza topped 500, many of them civilians, and the Israeli army said 18 of its soldiers had been killed, its heaviest losses in eight years.
Egypt has been a mediator in past Israel-Palestinian conflicts and has taken the lead in trying to broker a truce between Israel and its Islamist foe Hamas which dominates the Gaza Strip.
A first proposal Egypt made early last week was accepted by Israel but snubbed by Hamas, which said it was not consulted and demanded a raft of changes.
The Islamist movement wants Israel to agree to an end to its blockade of Gaza and the release of scores of prisoners before it will agree to halt its attacks, the latest of which saw 10 terrorists infiltrate southern Israel early on Monday.
It has received support from Qatar and Turkey, both considered to be Western allies that also have close relations with radical Islamists.
Kerry will seek "an immediate cessation of hostilities based on a return to the November 2012 ceasefire agreement," the White House said, stressing the need to protect civilian life both "in Gaza and in Israel."
It was referring to the Egyptian-brokered truce that ended the last major bout of fighting in and around Gaza, during Israel's Operation Pillar of Defense.
That ceasefire stipulated that Israel ease its blockade of Gaza's border crossings and coast, something Hamas complains was never fulfilled.
Hamas leader Khaled Meshaal and Palestinian Authority head president Mahmud Abbas were to hold talks in Qatar on the truce negotiations on Monday, a day later than planned.
The 2012 truce was brokered when Egypt was ruled by now-ousted Islamist president Mohamed Morsi who had close relations with Hamas.
His successor President Abdel Fattah al-Sisi – who as army chief deposed Morsi – has taken a hard line with Hamas, accusing it of helping Egyptian Islamist terrorists.
Kerry has publicly defended Israel but appeared to criticize the US ally in candid remarks caught on an open microphone between television interviews on Sunday.
Kerry was heard talking about Israeli soldiers killed in Gaza to a State Department official identified as Jonathan Finer just before appearing on the "Fox News Sunday" political talk show.
"I hope they don't think that's an invitation to go do more," Kerry says. "That better be the warning to them."
A frustrated Kerry then says: "It's a hell of a pinpoint operation, it's a hell of a pinpoint operation," in apparent frustration over the civilian toll in the Israeli operation.
"We've got to get over there," Kerry is heard saying on the Sunday recording. "I think it's crazy to be sitting around. Let's go."
The UN Security Council held urgent talks on the conflict late Sunday, expressing "serious concern" about the rising death toll and demanding "an immediate cessation of hostilities."
Ban urged Israel to "exercise maximum restraint" saying: "Too many innocent people are dying."
Sunday was the deadliest day of the conflict so far with more than 150 Palestinians killed in a blistering bombardment that left bodies lying in the streets and sent thousands of civilians fleeing their homes.
The Israeli army said 13 soldiers were also killed, its heaviest single day losses since 2006.