The civilian death toll in Gaza increased dramatically today, with reports of more than 40 Palestinians killed after missiles exploded outside a UN school where hundreds of people were sheltering from the continuing Israeli offensive.

Two Israeli tank shells struck the school in Jabaliya refugee camp, spraying shrapnel on people inside and outside the building, according to news agency reports.

A United Nations official in Gaza said the school was clearly marked with a UN flag and its location had been reported to Israeli authorities. John Ging, director of operations in Gaza for UNRWA, the United Nations Relief and Works Agency, said that three artillery shells landed at the perimeter of the school where 350 people were taking shelter. “Of course it was entirely inevitable if artillery shells landed in that area there would be a high number of casualties,” he said.

Asked whether there were Hamas militants in the area at the time of the attack, Ging said it was the scene of clashes “so there’s an intense military and militant activity in that area.” He said UN staff vetted Palestinians seeking shelter at their facilities to make sure militants were not taking advantage of them. “So far we’ve not had violations by militants of our facilities,” he said. Ging called for an independent investigation of the strikes near UN facilities.

Israel continues to insist that the bulk of those killed are Hamas and Islamic Jihad fighters, although its claim to be going to extraordinary lengths to target only “terrorists” has been undermined by one of its own tanks firing on a building being used by Israeli troops, killing four.

The sharp spike in the number of civilian casualties came as Israeli troops and tanks moved into Gaza’s second largest city, Khan Younis, for the first time today, supported by intensive artillery strikes as the military pledged to press on with its attack.

Nine Israelis, including three civilians hit by rocket fire, have been killed in the conflict. At least five rockets fired from Gaza landed in Israel today, including one that hit the town of Gadera, 17 miles from Tel Aviv, police said. A three-year-old girl was wounded.