Al-Huriya News Agency -  At least 15 Palestinians were killed and about 90 injured early on Wednesday when a UN school sheltering displaced people was hit by shells during a second night of relentless bombardment that followed an Israeli warning of a protracted military campaign.

Pierre Krahenbuhl, commissioner-general of the UN agency for Palestinian refugees, UNRWA, condemned “in in the strongest possible terms this serious violation of international law by Israeli forces”.

He said in a statement: “Last night, children were killed as they slept next to their parents on the floor of a classroom in a UN-designated shelter in Gaza. Children killed in their sleep; this is an affront to all of us, a source of universal shame. Today the world stands disgraced.

“We have visited the site and gathered evidence. We have analysed fragments, examined craters and other damage. Our initial assessment is that it was Israeli artillery that hit our school, in which 3,300 people had sought refuge. We believe there were at least three impacts.

“It is too early to give a confirmed official death toll. But we know that there were multiple civilian deaths and injuries including of women and children and the UNRWA guard who was trying to protect the site. These are people who were instructed to leave their homes by the Israeli army.”

It was the sixth time that UNRWA schools had been struck, he added. “Our staff, the very people leading the humanitarian response are being killed. Our shelters are overflowing. Tens of thousands may soon be stranded in the streets of Gaza, without food, water and shelter if attacks on these areas continue.”

At the school, Assad Sabah said he and his five children were huddling under desks in one of the classrooms because of the constant sound of tank fire throughout the night.

“We were scared to death,” he told the Associated Press. “After 4.30am, tanks started firing more. Three explosions shook the school. One classroom collapsed over the head of the people who were inside.”

A spokeswoman from the Israel Defence Forces said that its initial inquiries showed that “Hamas militants fired mortar shells from the vicinity of the school, and [Israeli] soldiers responded by firing towards the origins of the fire”. An investigation was continuing, she added.

A UN source said there was no evidence of militant activity inside the school.

The shelling of the school came as diplomatic attention was focussed on Cairo, where a delegation including the Palestinian president, Mahmoud Abbas, and representatives of Hamas and Islamic Jihad, the main militant factions in Gaza, was due to take part in ceasefire talks. A key issue was whether the Gaza-based factions and their armed wings accepted the authority of the delegation.

The Israeli security cabinet was also due to meet on Wednesday afternoon and would consider any progress made in Cairo. Israel’s political and military leaders face crucial decisions on whether to press deeper into Gaza once the cross-border tunnels have been located and destroyed, or whether to accept a “quiet for quiet” deal. “The next 24-72 hours will be critical,” said a diplomatic source.

The last two nights have seen the most fierce bombardment in this Gaza offensive, with inense air strikes, tank shelling and bombardment from Israeli gunboats. In 23 days more than 1,240 Palestinians, most of them civilians, have been killed. On the Israeli side 53 soldiers and three civilians have been killed.

The shelling of the UN school followed an incident last week when another UN school in Beit Hanoun was hit as the playground was filled with families awaiting evacuation amid heavy fighting. Israel denied it was responsible for the deaths, saying a single “errant” shell fired by its forces hit the school playground, which was empty at the time.