The Pentagon said it was sending the aircraft carrier USS Gerald R. Ford and its accompanying warships to the eastern Mediterranean, while boosting fighter aircraft squadrons in the region.

The United States, a major supplier of arms to Israel, has moved quickly to affirm its backing for Israel after Saturday's surprise attack from the Gaza Strip, vowing "rock solid" support and warning other parties to stay out of the conflict.

Biden spoke to Israeli Prime Minister Benjamin Netanyahu and "conveyed that additional assistance for the Israeli Defense Forces is now on its way to Israel with more to follow over the coming days," the White House said.

"The leaders also discussed ongoing efforts to ensure that no enemies of Israel believe they can or should seek advantage from the current situation," it said in a statement.

The US president further "pledged his full support for the Government and people of Israel in the face of an unprecedented and appalling assault by Hamas terrorists."

The Israeli-Palestinian conflict's worst escalation in decades has claimed more than 600 lives on the Israeli side, the government press office said, while Gaza officials reported at least 370 deaths in Israeli air strikes.

Israel's Netanyahu has steeled his shocked nation for what he called a long war ahead, with tens of thousands of Israeli forces already deployed to battle holdout Hamas forces and prepare a possible Gaza ground offensive.

- 'Ironclad support' -

The conflict has also stoked fears of a wider conflagration. Lebanon's Iran-backed Hezbollah movement said Sunday it had fired shells and missiles at Israeli positions.

With tensions rising, US Defense Secretary Lloyd Austin said he was sending forces to "bolster regional deterrence efforts" after discussions with Biden.

As well as the USS Gerald R. Ford carrier, this deployment includes a guided missile cruiser and four guided misile destroyers, Austin said in a statement.

"The United States government will be rapidly providing the Israel Defense Forces with additional equipment and resources, including munitions," Austin added.

The movement of US ships and planes and the aid to Israel "underscores the United States' ironclad support for the Israel Defense Forces and the Israeli people."

US Secretary of State Antony Blinken said earlier that Biden ordered his administration to give Israel "everything it needs in this moment to deal with the attacks from Hamas."

A number of Americans may also have been take hostage, Blinken said.

"We have reports that several Americans were killed. We're working overtime to verify that. At the same time, there are reports of missing Americans and there again, we're working to verify those reports," Blinken said.

He told ABC that "this is a massive terrorist attack that is gunning down Israeli civilians in their towns, in their homes, and as we've seen so graphically, literally dragging people across the border with Gaza.

"So, you can imagine the impact this is having throughout Israel. And the world should be revolted at what it has seen."