Israel pounded Gaza from the air as troops also pressed a ground assault, sending the Palestinian death toll soaring to 337.

As world diplomats frantically sought ways to end 12 days of bloodshed in and around Gaza, Israel warned it was ready to intensify a ground operation to destroy a network of cross-border tunnels used by militants to infiltrate the Israeli south.

US President Barack Obama urged Israel to do everything necessary to curb the high civilian death toll, saying Washington was “deeply concerned about the risks of further escalation”.

But even as the operation gathered force, Palestinian commandos succeeded in infiltrating Israel, sparking a deadly skirmish with an army patrol, which ended with a militant dead and two soldiers wounded, as Gaza’s bloodiest conflict since 2009 showed no signs of letting up.

And Israel’s Chief-of-Staff, Lieutenant General Benny Gantz, said the army was “expanding the ground phase of the operation”, warning there would be “moments of hardship”, alluding to the possibility of further Israeli casualties.

So far, the violence which erupted on July 8, has claimed the lives of 337 Palestinians, and three Israelis, one of whom was killed on Saturday when a rocket hit a Bedouin encampment in southern Israel.

In Gaza, after a relative lull on Friday, violence picked up again in the evening, with intensifying tank shelling and air strikes killing more than a dozen people.

And on Saturday, at least 41 people were killed, including two six-year-olds and a toddler, emergency services spokesman Ashraf al-Qudra said.

The increasing number of children killed in the conflict is causing a growing outcry, with a joint statement from the NGOs War Child and Defence for Children International saying more children had been killed than militants.

Figures provided by the UN children’s agency, UNICEF, indicate 73 of the victims were under the age of 18.

The UN agency for Palestinian refugees UNRWA has so far opened 44 of its schools to shelter those fleeing in the most heavily-bombarded areas.

So far, more than 50,000 Gazans have sought sanctuary at UN institutions, the agency said.

Meanwhile, UN chief Ban Ki-moon was to leave for the region on Saturday to help Israelis and Palestinians “end the violence and find a way forward”, the agency said.