Washington (AFP) - The United States confirmed it had restocked Israel's supplies of ammunition, hours after finally sharpening its tone to condemn an attack on a United Nations school in Gaza.

But while both the White House and the State Department condemned the shelling of the UN-run school in the Jabaliya refugee camp in northern Gaza in which at least 16 Palestinians were killed, neither would assign blame to staunch US ally Israel.

"Obviously nothing justifies the killing of innocent civilians seeking shelter in a UN facility," deputy State Department spokeswoman Marie Harf acknowledged, in some of the toughest US comments since the start of the 23-day fighting in the Gaza Strip.

"Innocent Palestinians seeking refuge in these schools should not have shells dropped on them, should not come under attack."

The UN agency for Palestinian refugees UNRWA said Israeli forces had hit the school, which had been sheltering some 3,300 Gazans.

But despite heated exchanges with reporters, Harf stressed that "we don't know for certain who shelled this school, we need to get all the facts."

National Security Council spokeswoman Bernadette Meehan also condemned "those responsible for hiding weapons in United Nations facilities in Gaza" and warned of rising fears that thousands of Palestinians who have been told by Israel to leave their homes increasingly had nowhere to go in the blockaded narrow coastal strip.

US officials also warned that patience with "crazy" Israeli criticism of would-be-peacemaker John Kerry had snapped.

- New ammunition for Israel -

The Pentagon confirmed the Israeli military had requested additional ammunition to restock its dwindling supplies on July 20, with the US Defense Department approving the sale just three days later.

"The United States is committed to the security of Israel, and it is vital to US national interests to assist Israel to develop and maintain a strong and ready self-defense capability," Pentagon spokesman Rear Admiral John Kirby said in a statement.

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"This defense sale is consistent with those objectives."

Two of the requested munitions came from a little-known stockpile of ammunition stored by the US military on the ground in Israel for emergency use by the Jewish state. The War Reserve Stockpile Ammunition-Israel is estimated to be worth $1 billion.

The decision to provide ammunition to Israel could fuel controversy, coming just as Washington expresses growing concern about the deaths of more than 1,300 Palestinians, most of them civilians, since the Israeli operation began on July 8.

Kirby said Pentagon chief Chuck Hagel told his Israeli counterpart that the United States was concerned about the deadly consequences of the spiraling conflict, including a "worsening humanitarian situation" in Gaza, and called for a ceasefire and end to hostilities.

He also renewed calls for the disarmament of Gaza's Hamas rulers and "all terrorist groups."

Relations between Israel and its staunch ally the United States have plunged in recent days after Kerry returned from a mission to the Middle East to try to broker a ceasefire between the Israelis and Hamas militants.

Anonymous Israeli officials have hit out at Kerry's truce proposal, calling it "a strategic terrorist attack" and criticizing it for being a "Hamas wish-list" including moves to lift a long-standing Israeli blockade of Gaza while failing to address Israel's security concerns, such as Hamas rocket fire and a network of underground tunnels.

And on Tuesday a fabricated transcript of a call between US President Barack Obama and Israeli Prime Minister Benjamin Netanyahu went viral on social media.

- Out to hurt ties? -

Stressing the "unprecedented" US support for the Jewish state, Harf hit out at Israeli elites' "offensive and absurd" claims that Kerry backs Hamas.

She rubbished the fake transcript as "complete crap," adding "there's clearly people... who are putting out false and defamatory and absurd information."

"I don't know what else you can assume about the intentions except that they’re designed to hurt our relationship," she added.

Washington, which has provided billions in military aid to Israel, including funding the Iron Dome shield protecting the country from Hamas rockets, was "very committed" to the security of the Jewish state, which is "why these vicious attacks on the secretary are just crazy," she added.

And US lawmakers are working on a package of additional military support from Washington to commit $225 million for the Iron Dome missile defense shield.

More than 100 people died in Israeli strikes across Gaza Wednesday, medics said, including 17 at a crowded marketplace, sending the Palestinian toll from the 23 days of fighting to 1,363.

On the Israeli side, the conflict has cost the lives of 56 Israeli soldiers, and two civilians.