Fifteen people were killed Thursday when Israeli fire hit a UN shelter in Gaza, as the Palestinian toll in the 17-day conflict rose to 798, medics said.
Emergency services spokesperson Ashraf al-Qudra said at least 15 people had been killed and 200 wounded by Israeli shelling of a school run by the UN agency for Palestinian refugees (UNRWA) in the northern town of Beit Hanun, where hundreds of civilians had sought refuge from the violence.
He gave no immediate details of those killed, but an AFP correspondent reported that a mother and her one-year-old infant were among the dead brought into a nearby mortuary.
Earlier Thursday, US airlines lifted a flight ban to Israel, with other international airlines expected to follow suit.
The US national aviation agency had imposed the restriction on Tuesday after a rocket hit a house very close to the runways, in a move mirrored by Europe.
It was renewed late on Wednesday, prompting Hamas, the de facto power in Gaza, to hail the suspension of Tel Aviv flights as a "great victory for the resistance."
Shortly afterwards, the US agency rescinded the move.
Kerry -- who is in Egypt, which has drafted a truce proposal for the Israel-Hamas conflict -- spoke by phone with the foreign ministers of Qatar and Turkey, a US official said.
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Two soldiers run towards an Israeli tank before firing 155mm shells towards targets in the Gaza Strip from their position near Israel's border with the Palestinian enclave. (AFP photo)
Hamas chief Khaled Meshaal is based in Qatar, while Turkey's Islamist-oriented Prime Minister Recep Tayyip Erdogan has harshly criticised Israel's assault on Hamas-ruled Gaza as well as Egypt's role in trying to clinch a ceasefire.
A US official said Kerry was hoping Qatar and Turkey would use their influence to encourage Hamas to accept a ceasefire plan, which the Islamist group has so far rejected.
Meshaal vowed late on Wednesday there would be no end to the fighting without the lifting of Israel's eight-year blockade on Gaza.
Despite Hamas's intransigence, the skies over southern Israel remained quiet for seven hours, the army said, in what was the calmest night since the Israeli operation began on July 8.
Since 5:00 am (0200 GMT), just six projectiles struck the south, while another three were intercepted, a spokeswoman said.
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Khuzaa under fire
There was no let-up to the violence in Gaza, however, with most of Thursday's 51 victims killed in and around Khuzaa, a flashpoint area east of Khan Yunis which has been the site of intensive fighting since Tuesday.
Gaza's health ministry issued a call for international protection for civilians in the area, saying anyone leaving home was being targeted by Israeli fire.
On Wednesday, the Red Cross and Palestinian ambulances managed to evacuate 150 people from the area following negotiations with both sides, and another convoy of 10 ambulances entered the area early on Thursday, an ICRC spokesperson told AFP.
In Jabaliya refugee camp, residents gathered at first light to examine the damage after an air strike destroyed houses and a mosque.
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"The blast was strong and it destroyed houses and the mosque, it was an F16," Faeq Hussein told AFP as two men struggled over piles of rubble and twisted metal to try to salvage a red armchair.
"All this area is a refugee camp, the houses are very close together and everyone was affected. We are all civilians and this mosque had nothing to do with the rocket fire," he said, the buildings around him peppered with shrapnel.
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The Gaza-based Palestinian Centre for Human Rights has said more than 80 percent of the casualties were civilians, and a quarter of them children, triggering growing international alarm over the civilian body count.
"We are gravely concerned by the ongoing heavy level of civilian causalities," British Foreign Secretary Philip Hammond said at a press conference in Jerusalem with Israeli Prime Minister Benjamin Netanyahu, at which he also defended Israel's right to self-defence in the face of a conflict triggered by Hamas rocket fire.
Netanyahu said Israel was doing everything it could to minimise casualties, pinning the blame on Hamas for its "grotesque (and) inhuman" use of civilians as human shields.
Hammond then flew to Cairo where he was due to hold talks with Egyptian President Abdel Fattah al-Sisi.
UN chief Ban Ki-moon was also in Egypt and Israel earlier this week, in the hope of hammering out a regional truce deal, with Kerry acknowledging there had been "some progress in moving toward that goal."
Two men flash the V-sign as they hold a Palestinian flag during a demonstration in Paris, to denounce Israel's military campaign in Gaza and to show their support to the Palestinian people. (AFP photo)
On a whirlwind visit, Kerry on Wednesday met Palestinian president Mahmud Abbas in the West Bank city of Ramallah then held a two-hour meeting in Tel Aviv with Netanyahu before flying back to the Egyptian capital.
So far, 32 Israeli soldiers and three civilians have died in the fighting, one of whom was a Thai farm labourer who was killed when a rocket struck the greenhouse where he was working in southern Israel.
Following his death, Bangkok demanded Israel "immediately" relocate 4,000 Thai nationals working near the Gaza Strip to areas safe from the fighting, a foreign ministry spokesman said.
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A Jewish worshipper takes part in a special prayer at the Western Wall in Jerusalem's Old City for the well-being of Israeli soldiers in Gaza. (Reuters photo)
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A Palestinian rescue convoy navigates rubble in Gaza City's Shijaiyah neighborhood, during a two hour temporary ceasefire. (AP photo)
Full coverage: Gaza under attack
(With inputs from Reuters and AFP)