While saying Israel has a right to defend itself, White House Deputy Press Secretary Eric Schultz said, "We've also been very clear that Israel needs to do more to live up to its own standards to limit civilian casualties."

It was a sentiment echoed by U.S. State Department spokeswoman Marie Harf, who also said Israel is being put in a tough situation because "Hamas is hiding rockets in places to prevent the Israelis from going after them."

"So look, Hamas is taking steps here that put civilians at risk. But we do believe the Israelis need to do more," Harf told reporters during a briefing.

More than 1,300 Palestinians have been killed since the conflict between Israel and Hamas began on July 8, the Palestinian Ministry of Health in Gaza said.

The number of militants killed is unclear, but the United Nations estimates that 70% to 80% of the dead are civilians.

Three more Israeli soldiers were killed Wednesday, bringing the total to 56 dead. Three civilians also have been killed in Israel.

While there were calls on Israel to do more to protect civilians, the United States also agreed to Israel's request to resupply it with several types of ammunition, a U.S. defense official told CNN on condition of anonymity. It's not an emergency sale, the official said.

Among the items being bought are 120mm mortar rounds and 40mm ammunition for grenade launchers, the official said. Those will come from a stockpile the United States keeps in Israel, which is worth more than $1 billion.

U.N. facility struck

Sanaa Abugerad was among some 3,000 Palestinians at a school-turned-shelter when it was hit by artillery early Wednesday.

"We saw the shells when they hit, and shrapnel was falling like rain," she told CNN. "I was so scared and the school filled with smoke. We poured water in our eyes just to see."

The United Nations blamed Israel for the attack, with U.N. spokesman Chris Gunness saying his group's initial assessment indicates Israeli artillery hit the school. The number of deaths was not immediately clear, Gunness said. The Palestinian Health Ministry said 20 were killed.

But Israel said a group of militants fired at Israeli soldiers from the vicinity, and the soldiers "responded by firing at the origin of the fire," a military spokesman said.