Israel-Gaza conflict: US condemns shelling of UN school in Gaza Strip which killed 15

The US did not say who was responsible for the shelling

Palestinians youths inspect a destroyed classroom of a UN school in Jabalia, northern Gaza Strip, 30 July 2014. Dozens of Palestinians were killed in another night of heavy airstrikes and artillery on the Gaza Strip overnight, including the deadly shellin ( EPA/MOHAMMED SABER)

The United States has condemned the shelling of a UN school in the Gaza Strip that was sheltering displaced Palestinians, in its sharpest criticism of the three-week conflict.

At least 15 people were killed and 90 people were wounded by tank shells at the school in Jebaliya, before dawn on Wednesday, Gaza health officials said.

Adnan Abu Hasna, a spokesman for the UN Relief and Works Agency which is sheltering more than 200,000 people displaced by the fighting, 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 said. "After 4:30 a.m., tanks started firing more. Three explosions shook the school."

While Gaza officials said Israel had shelled the school, the Israeli military said it fired back after soldiers were targeted by mortar rounds launched from the vicinity of the school.

In a statement in which the US did not say who was responsible for the shelling, White House spokeswoman Bernadette Meehan said the US is also “extremely concerned” that thousands of Palestinians aren't safe in UN-designated shelters in Gaza.

Meehan added the US also condemns those responsible for hiding weapons in UN facilities in Gaza, and said the escalation shows the need for a cease-fire as soon as possible.

"We are extremely concerned that thousands of internally displaced Palestinians who have been called on by the Israeli military to evacuate their homes are not safe in UN designated shelters in Gaza.

"We also condemn those responsible for hiding weapons in United Nations facilities in Gaza," she said.

The statement came amid an escalation of the violence in Gaza, where an Israeli airstrike hit a crowded Gaza shopping area on Wednesday, killing at least 15 people and wounding more than 150.

Israel and Gaza's Hamas rulers have stepped up attacks after more than three weeks of fighting that has killed more than 1,300 Palestinians and more than 50 Israelis.

Earlier at the UN in New York, top officials also expressed shock and outrage at the attack on the UN school, with Deputy Secretary-General Jan Eliasson repeating the UN's call for an immediate cease-fire.

Additional reporting by agencies