Earlier, the Israeli military said it was calling up some 16,000 reservists at short notice, bringing the total number of troops mobilised during the conflict to 86,000. A spokesperson said the troops are joining the operation to allow troops on the ground to rest.
Meanwhile, the White House expressed concern at the deaths in Jabalya and other UN-run shelters shelled during the clashes.
"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," said National Security Council spokeswoman Bernadette Meehan.
This came as a US defence official claimed to CNN Washington that has also agreed to resupply Israel with several types of ammunition.
The UN Relief and Works Agency (UNRWA) says it is at "breaking point" with over 225,000 civilians seeking refuge in 86 of their shelters.
UNRWA spokesman Chris Gunness described the shelling of the UN school in Jabaliya, in which 19 people were killed and 125 wounded,"a source of international shame".
The Israeli government spokesman Mark Regev told the BBC Israel would apologise if it was determined that their fire was responsible for the Jabaliya school strike.
"We have a policy - we don't target civilians," he said. "It's not clear to us that it was our fire but we know for a fact there was hostile fire on our people from the vicinity of the school."
Attacks in the Gaza strip are continuing, with witnesses saying that munitions struck the Omar Ibn al-Khatab mosque next to a UN school in the northern town of Beit Lahiya on Thursday. wounding three.
