US President Joe Biden said Americans were likely among those held by Hamas
US President Joe Biden said Americans were likely among those held by Hamas AFP

President Joe Biden said Monday that Americans are likely being held hostage by Palestinian militant group Hamas after its surprise attack on Israel, and that at least 11 US citizens were killed in the assault.

Hamas dragged off about 150 hostages in its weekend onslaught on southern Israel from Gaza, an attack that a senior US defense official said involved "ISIS-level savagery".

Hamas threatened on Monday to kill the hostages if retaliatory Israeli air strikes continue "targeting" Gaza residents without warning.

The threat came after Israel imposed a total siege on the Gaza Strip, cutting off the water supply, which sparked UN fears of an increasingly dire humanitarian situation.

Biden said that Washington, which has moved an aircraft carrier closer to Israel in a show of support, was working with its key ally on efforts to recover the hostages after the "heart-wrenching" attacks.

"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.

"I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts."

Biden added that the safety of American citizens at home or abroad was his "top priority as president."

Reeling from the Palestinian Islamist group's unprecedented ground, air and sea attacks, Israel has counted 800 dead and launched a withering barrage of strikes on Gaza that have raised the death toll there to 687.

The United States sent its largest aircraft carrier and five other warships near Israel in a show of support on Sunday, while boosting fighter aircraft squadrons in the region.

A senior US defense official said Monday said 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 United States has meanwhile warned other parties not to get involved, as fears of a regional conflagration grow.

Lebanese militant group Hezbollah should not make the "wrong decision" of opening a second front against Israel as it battles Hamas, the defense official said.