President Joe Biden said Monday that 11 Americans are dead and others are likely being held hostage by the Palestinian militant group Hamas after a surprise attack on Israel using what a US official called "ISIS-level savagery."

As fears grew that the conflict could turn into a regional conflagration, the White House said there was no intention to put US boots on the ground but warned Iran and other actors not to get involved.

Biden and the leaders of France, Germany, Italy, and Britain meanwhile pledged their support for their ally Israel to defend itself as the death toll there rose to 800, while in Gaza, 687 people have died.

Hamas dragged off about 150 hostages in its weekend onslaught on southern Israel from Gaza and has threatened to kill them if retaliatory Israeli air strikes continue "targeting" Gaza residents without warning.

Biden said that Washington, which has moved its biggest aircraft carrier and other warships closer to Israel in a show of support, was working with its key ally on "hostage recovery efforts."

"Sadly, we now know that at least 11 American citizens were among those killed — many of whom made a second home in Israel," Biden said in a statement after meeting his national security team at the White House.

"While we are still working to confirm, we believe it is likely that American citizens may be among those being held by Hamas," he added.

The White House would be lit in the blue and white colors of the Israeli flag on Monday night in a sign of US "ironclad" support.

Biden was due to deliver further remarks on the "terrorist attacks in Israel" on Tuesday, the White House said.

Massacred

A senior US defense official said Monday that the hostages were the subject of intense engagement with the Israelis.

"There are continual conversations across the US government with all of our Israeli counterparts about the situation of hostages that Hamas is holding in Gaza," the official told journalists.

The US official also likened the attacks by Hamas to the notorious brutality of the Islamic State jihadist group, which is known by the acronym ISIS.

"This is ISIS-level savagery that we have seen committed against Israeli civilians — houses burned to the ground, young people massacred at music festivals," the official said.

The White House said that while it was sending fresh military aid to Israel to deter its "enemies," there was no question for now of the United States getting involved in the conflict.

"There's no intention to put US boots on the ground," National Security Council spokesman John Kirby told reporters.

Kirby said that there was no "hard or tangible evidence" linking Iran to the Hamas attack, even if there was a "degree of complicity" due to Tehran's support for the group.

Iran has denied any involvement.

Biden, French President Emmanuel Macron, German Chancellor Olaf Scholz, Italian Prime Minister Giorgia Meloni, and British Prime Minister Rishi Sunak issued a joint statement saying they "support Israel in its efforts to defend itself," the White House said.

An initial version of the statement from the French presidency warned Iran not to "spread the conflict beyond Gaza," but the reference to the Islamic Republic, which did not appear in the White House or Downing Street versions, was later removed.