The United States confirmed it had restocked Israel's supplies of ammunition, hours after issuing a strong condemnation of an attack on a United Nations school in Gaza.

The Israeli military requested additional ammunition to restock its dwindling supplies on July 20, the Pentagon said. The US Defense Department approved 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.

"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.

Kirby stressed that those munitions had been in WRSA-I stock for "a few years, well before the current crisis."

"All stocks in WRSA-I, as required by law, are 'in excess to US requirements," he added.

"Issuing munitions from the WRSA-I stockpile was strictly a sourcing decision and White House approval was not required."

But Kirby did not indicate whether the White House was involved in the decision to supply the other ammunition.

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, and called for a ceasefire and end to hostilities.

In a telephone call with Israeli Defense Minister Moshe Yaalon, Hagel expressed "the United States' continued concern about the rising number of Palestinian civilian deaths and loss of Israeli lives, as well as the worsening humanitarian situation in Gaza."

Hagel "stressed the need for a humanitarian ceasefire that ends hostilities and leads to a permanent cessation of hostilities," the spokesman added.