The UN Security Council has called for an immediate ceasefire in Gaza after holding an emergency closed-door meeting on the crisis.
The talks came after the UN chief described Israel’s shelling of a Gaza City suburb as “an atrocious action”.
A total of 501 people have now died in Gaza during Israel’s operation against Hamas militants, according to Gaza’s Ministry of Health, with 3,135 injured.
US Secretary of State John Kerry is due in Egypt to discuss the crisis.
He will meet political leaders in Cairo on Monday, as efforts to broker a ceasefire continue.
On Sunday Gaza saw its deadliest day since the Israeli offensive began on 8 July, with 13 Israeli soldiers and more than 100 Palestinians killed.
Twenty Palestinians have been killed in strikes so far on Monday, the Gaza health ministry says, 16 of them in southern Gaza.
Sunday’s late-night UN Security Council session was convened at the request of Jordan, which is understood to have proposed a strongly worded draft resolution for consideration.
However, members could only agree on “elements to the press,” the weakest form of Security Council action, says the BBC’s UN correspondent Nick Bryant.
Eugene Gasana, Rwanda’s ambassador to the UN, said members voiced alarm at the escalation of violence during a “sobering session”.
They also expressed “serious concern” at rising casualty numbers and called for “the respect of international humanitarian law, including protection of civilians,” Mr Gasana added.
More than 60 Palestinians alone were killed during heavy shelling in the Shejaiya district of Gaza, with Palestinian President Mahmoud Abbas calling it “a massacre”.
He called for urgent talks, saying the “situation is intolerable” in Gaza and describing the Israeli attacks as “crimes against humanity.”
Israeli Prime Minister Benjamin Netanyahu vowed to continue operations “as much as we need to” despite the number of Israeli soldiers killed rising to 18 at the weekend.
Two of the soldiers killed were American citizens, US state department spokesperson Jen Psaki said.
Sunday’s death toll for Israel’s military is higher than that sustained during the entire three-week duration of Operation Cast Lead in 2008-2009, the last time Israel sent troops into Gaza.
The deaths of so many soldiers on a single day will shock Israeli society, the BBC’s Chris Morris reports from southern Israel.
Hamas said on Sunday evening that it had captured a member of the Israel Defense Forces (IDF), prompting celebrations on the streets of Gaza and West Bank.
However Israel’s UN ambassador Ron Prosor denied the claim.
“There’s no kidnapped Israeli soldier and those rumours are untrue,” Mr Prosor said.
The UN says 83,695 people have now been displaced in Gaza and that the figure is “rising all the time”.
There were scenes of panic in Shejaiya, with thousands of residents fleeing.
In an interview with BBC Arabic, Mr Netanyahu called Shejaiya a “terror stronghold” and a centre for rocket attacks on Israel.
Mr Netanyahu said Israeli troops had no choice but to enter densely populated areas and that they had asked civilians to leave.
The Israel Defense Forces (IDF) sent ground troops into Gaza on Thursday after days of heavy air and naval barrages failed to stop rocket fire from Gaza.
Two Israeli civilians have died since Israel’s Operation Protective Edge began.
Israel says the ground operation is necessary to target Hamas’ network of tunnels, which have been used by militants to infiltrate Israel and carry out attacks.