President Joe Biden ordered US ships and warplanes to move closer to Israel in a show of support on Sunday, while sending fresh military aid after attacks by the Palestinian militant group Hamas.

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.

US Central Command confirmed Sunday afternoon that ships and planes had begun moving to their new posts.

At least four American citizens are known to have been killed in the violence so far, according to Senate Majority Leader Chuck Schumer.

The White House has moved quickly to affirm US 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 on Sunday 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."

Hamas later accused the United States, a major supplier of arms to Israel, of "actual participation in the aggression against our people" by moving the aircraft carrier.

The Israeli-Palestinian conflict's worst escalation in decades has claimed more than 700 lives on the Israeli side, according to the military, while Gaza officials reported at least 400 deaths in Israeli air strikes.

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 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 missile 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," added Austin, who also spoke with his Israeli counterpart on Sunday.

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."

The United States on Sunday afternoon led calls for condemnation of the Hamas assault on Israel at the United Nations Security Council.

"I expect to hear from the other Council members very strong condemnation of these heinous acts of terrorism committed against the Israeli people and their government," senior US diplomat Robert Wood told reporters.

'World should be revolted'

A US National Security Council spokesperson had Sunday evening confirmed the deaths of a number of Americans, without giving any further details.

'But in a statement after a Senate briefing with senior national security and State Department officials, the Majority leader Schumer said: "The Administration told us that they know of four thus far, but, sadly, we know the toll will rise.

Earlier Sunday, US Secretary of State Antony Blinken said Americans may also have been taken hostage in the attacks.

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."

State and local authorities throughout the United States -- including in New York, Los Angeles, Miami and Houston -- said there would be increased security presence at US synagogues in the coming days, though there was no specific threat.

© Agence France-Presse