GAZA CITY, Palestinian Territories (AFP) ― The U.N. Security Council called for an “immediate cease-fire” as Israel pressed on with a blistering assault on Gaza on Monday taking the Palestinian death toll above 500.

As world efforts to end the fighting gathered pace, Israel said it killed 10 Hamas militants in an early morning gun battle in southern Israel after they used a tunnel to get across the border.

U.S. President Barack Obama echoed the call for an immediate cease-fire in a telephone conversation with Israeli Prime Minister Benjamin Netanyahu early Monday, as Secretary of State John Kerry headed to the region to join truce efforts.

A fresh Israeli air strike killed a family of nine, including seven children, in the southern city of Rafah early Monday, emergency services said.

It came after after more than 150 Palestinians were killed on Sunday, the deadliest day so far of an offensive now in its 14th day.

The Israeli army said 13 soldiers had been killed inside Gaza on Sunday, raising to 18 the number of soldiers killed since a ground operation began late on Thursday.

They were the army’s heaviest losses in eight years and left Israelis too in mourning.

A weekend of incessant Israeli bombardment by land, sea and air sent thousands of terrified Palestinian civilians fleeing their homes.

Additional deaths across Gaza Monday pushed the Palestinian toll to at least 508, not counting the 10 militants who sneaked into southern Israel through a cross-border tunnel.

“Two terrorist squads infiltrated Israel through a tunnel from northern Gaza. IDF aircraft struck one; IDF forces killed 10 terrorists,” the army said.

Several Israeli soldiers were wounded in the exchange, which took place near kibbutz Niram close to the Erez crossing, army radio said.

The attack was claimed by Hamas armed wing the Ezzedine al-Qassam Brigades, which said it had carried out “an operation behind enemy lines in response to the massacre in Shejaiya.”

It was referring to a neighborhood between Gaza City and the Israeli border, where a ferocious Israeli bombardment, which began early Sunday, killed at least 72 Palestinians.

Late on Sunday, the Hamas armed wing said it had captured an Israeli soldier it named as Shaul Aaron.

The army said it was checking the reports, but Israel’s U.N. Ambassador Ron Prosor said the claim was untrue.

Hamas militants held Israeli soldier Gilad Shalit for more than five years until his release in a 2011 exchange for more than 1,000 Palestinian prisoners.

The U.N. 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.”

Palestinian U.N. envoy Ryad Mansour voiced frustration with what he termed the world body’s failure to take a strong stand.

Several hours later, Obama telephoned Netanyahu to echo calls for an “immediate cease-fire.”

U.N. chief Ban Ki-moon, who is currently in the region to bolster truce efforts, also urged Israel to “exercise maximum restraint,” saying: “Too many innocent people are dying.”

Hamas leader Khaled Meshaal and Palestinian president Mahmud Abbas were to hold talks in Qatar on Monday, a day later than planned.

So far, cease-fire proposals have been rejected by Hamas, which has laid out a long list of demands it wants Israel to agree to, including an end to its blockade of Gaza and the release of scores of prisoners.

On the ground in Gaza, hundreds of people could be seen fleeing the northern area of Beit Hanun, a day after tens of thousands fled Shejaiya.

So far, some 83,000 people have taken shelter in some 60 schools run by the U.N. agency for Palestinian refugees UNRWA.

On Sunday, at least 72 people were killed in Shejaiya after hours of non-stop shelling reduced much of the neighborhood to rubble and left bodies lying in the streets.

The high civilian death toll in Gaza, including children, has sparked mounting international concern.

But Netanyahu has insisted that civilian casualties are the result of Hamas using innocents “as human shields.”

And he insisted Sunday that there was “very strong support” from the international community for Israel’s operation.

Although Israel said earlier Sunday it was expanding its ground operation to destroy tunnels used by militants to carry out cross-border attacks and fire rockets, Defense Minister Moshe Yaalon suggested it could end within days.