The IDF has said its targets in Gaza include rocket launchers, tunnels and the homes of senior Hamas leaders, which the IDF describes as "command centers."

But among the dead are 22 children and 15 women, including an 18-month-old baby and an 80-year-old woman, according to information from the Palestinian Health Ministry.

The Palestine Liberation Organization said Israeli bombs have hit civilian infrastructure, including a line that provides water to a refugee camp and a sewage plant.

The IDF has not responded to the accusations. It says it uses phone calls and drops empty shells on roofs -- what it calls "roof knocking" -- to warn civilians that airstrikes are imminent. But the approach doesn't guarantee their safety.

In one case, members of a family returned to a house in Gaza shortly after having been warned to evacuate it, Lerner, the Israeli military spokesman, said. They were caught in the airstrike.

He called their deaths a tragedy, saying, "This is not what the IDF does."

U.S. willing to help broker cease-fire

Hopes for a cease-fire appeared dim even as world leaders called for the two sides to stop the violence.

U.S. President Barack Obama and Israeli Prime Minister Benjamin Netanyahu spoke by phone.

"The United States remains prepared to facilitate a cessation of hostilities, including a return to the November 2012 cease-fire agreement," the White House said in a written statement, referring to the Egyptian-brokered deal that halted the previous Israel-Hamas conflict.

The President also condemned rocket attacks from Gaza and said the United States reaffirmed Israel's right to defend itself.

U.N. Secretary General Ban Ki-moon told reporters he has been reaching out to regional leaders in an attempt to help get the two sides to stop the violence soon.

"It is imperative not only to restore calm today, but to establish a political horizon for tomorrow," he said. "Without the prospect of an end to the conflict, the sides will grow ever more polarized."

'Prepared for all options'

There have been hints for days from some Israeli officials about the possibility of a ground offensive in Gaza, but there were questions about the government's appetite for such a conflict.

Netanyahu said Wednesday that the aerial offensive would be expanded and continue "until the firing at our communities stops and quiet is restored."

He didn't specify what the expansion of Operation Protective Edge would entail, saying that Israel's military "is prepared for all possibilities."

No Israelis have been killed so far by the hundreds of rockets fired toward southern Israel by Hamas, Islamic Jihad and other militant groups in Gaza. Some Israelis have been wounded by the attacks.

A CNN crew in Gaza was setting up a live shot when four rockets streaked overhead in the direction of Israel. Some in the crowd cheered. When told there were air raid sirens going off in Israel, there were more cheers.

The Israeli Defense Forces said early Friday that since the start of Operation Protective Edge, 548 rockets have been fired at Israel. The country's Iron Dome defense system has intercepted 118 of them, the IDF said.