Fifteen Palestinians were killed Thursday when an Israeli shell slammed into a UN shelter where hundreds of civilians had taken refuge, sending the Palestinian death toll in Gaza soaring to 788 with more than 4,000 injured despite world efforts to broker a ceasefire.
The strike hit a UN school sheltering some of the 100,000 Palestinians driven from their homes in search of a safe haven after weeks of Israeli aerial and ground offensive on Gaza.
The shell crashed down in the middle of the courtyard where people had set up camp, leaving the ground covered in bloodstains.
Gaza's emergency services said at least 15 people had been killed and more than 200 wounded, sending the Palestinian death toll from 17 days of fighting to 788.
UN chief Ban Ki-Moon said: "Many have been killed -- including women and children, as well as UN staff."
He said he was "appalled" by the news and "strongly condemned" the attack which he said "underscores the imperative for the killing to stop -- and to stop now".
Washington said it was "deeply saddened and concerned about the tragic incident", without explicitly blaming its ally Israel for the shelling.
"We again urge all parties to redouble their efforts to protect civilians," State Department spokeswoman Jen Psaki said in Cairo, where Secretary of State John Kerry is trying to negotiate a ceasefire.
Kerry reached out to Hamas allies Turkey and Qatar on Thursday as he sought to further regional efforts to broker an end to the bloodshed.
The US diplomat is seeking to garner support for an Egyptian-drafted proposal and an aide said he had spoken to his counterparts in Doha and Ankara in the hope they would use their influence to encourage Hamas to accept the plan.
Hamas has rejected the proposal so far, with its exiled leader Khaled Meshaal saying late Wednesday that there could be no halt to the fighting without an end to Israel's eight-year blockade on Gaza first.
The Egyptian plan calls for a ceasefire first and talks afterwards on a long-term solution to Gaza's problems.
Speaking after a meeting with Jordan's King Abdullah II, Palestinian president Mahmoud Abbas said Hamas should agree to the Egyptian ceasefire plan.
"After that, we will discuss what would happen in the future," he said.
There was no let-up to the Israeli offensive on Gaza, however, with most of Thursday's 93 victims killed in and around Khuzaa, a flashpoint area east of Khan Yunis which has been the site of intensive fighting since Tuesday.
But the biggest single strike was at the school in the north, where the UN agency for Palestinian refugees (UNRWA) said it had been trying to coordinate with the army over the evacuation of civilians, without success.
Although UNRWA did not immediately give its own toll, spokesman Chris Gunness said there were "multiple dead and injured" after an Israeli tank shell hit a UN school being used as a shelter.
An AFP correspondent saw nine bodies, including that of a year-old baby and his mother at a nearby mortuary.
"We've spent much of the day trying to negotiate or to coordinate a window so that civilians, including our staff, could leave," Gunness said.
"That was never granted... and the consequences of that appear to be tragic."
But Israeli army spokesman Lieutenant Colonel Peter Lerner cast doubt on whether the military was to blame, suggesting militants firing rockets near the school could have caused the deaths.
He also took issue with the claim that Israel had rejected a humanitarian truce around the school, saying it had implemented a four-hour window for evacuations.
The Gaza-based Palestinian Centre for Human Rights has said more than 80 percent of the casualties so far have been civilians, and a quarter of them children, triggering growing international alarm over the civilian body count.
"As this campaign goes on and the civilian casualties in Gaza mount, Western public opinion is becoming more and more concerned and less and less sympathetic to Israel," warned British Foreign Secretary Philip Hammond.
UN humanitarian chief Valerie Amos expressed deep concern about the mounting civilian casualties, saying it was "almost impossible" for Palestinians to shelter from Israeli air strikes in the densely-populated territory.
"It doesn't matter how hard Israel tries to minimise harm, this is an extremely overcrowded stretch of land," she said.
Meanwhile, US airlines on Thursday lifted a two-day suspension of flights to Israel, but not all international airlines followed suit, with Germanmy's Lufthansa extending its ban for a further 24 hours.
The ban was put in place on Tuesday after a rocket hit a house very close to the runways, with Hamas hailing the suspension of Tel Aviv flights as a "great victory".
So far, 32 Israeli soldiers and three civilians have died in the fighting.
Far-right hawk Reuven Rivlin was sworn in as Israel's 10th president on Thursday, replacing elder statesman Shimon Peres.
With the nation in mourning, the inauguration ceremony was scaled down, but Rivlin said it sent a "very clear message to our enemies: you have not overcome us and you will not do so".
*This story was edited by Ahram Online.