A UN school in Gaza’s Jabaliya refugee camp was shelled by the Israeli military early on Tuesday morning, killing 15 and leaving a further 90 wounded, according to Gaza health officials.

The school was being used by hundreds of Palestinians as a shelter to escape the fighting, said Adnan Abu Hasna, a spokesman for the UN aid agency, UNRWA.

The number of dead on the Palestinian side now stands at 1,250 while 56 Israelis have also been killed since the conflict began 23 days ago.

UNRWA also admitted in a press release that it had found Hamas rockets stored in one of its schools – the third such occasion in three weeks.

UNRWA Spokesman Chris Gunness said: “This is yet another flagrant violation of the neutrality of our premises. We call on all the warring parties to respect the inviolability of UN property.”

“We condemn the group or groups who endangered civilians by placing these munitions in our school,” he said.

This latest UN school shelling follows one of the most violent periods of the conflict. According to reports, 100 people have been killed in air strikes on the Gaza Strip in the past 24 hours.

Israeli Prime Minister Benjamin Netanyahu said that military action was likely to be “prolonged”.