The White House said Tuesday it was taking Hamas threats to execute hostages seriously, as US President Joe Biden prepared to deliver remarks about Israel's war with the militant group.

Biden has not spoken in public since Sunday, and while the 80-year-old has staunchly backed Israel, his response to a possible hostage crisis and threat of a regional war is under the microscope.

The president has said that Americans were likely among those abducted and taken to Gaza by the Palestinian militant group Hamas in its surprise attack Saturday on Israel, while at least 11 US citizens were dead.

Biden and Vice President Kamala Harris were due to speak to Israeli Prime Minister Benjamin Netanyahu on Tuesday morning after meeting their national security teams, the White House said.

With Netanyahu they would "discuss our support for Israel" and efforts to "deter other hostile actors from exploiting this attack on Israel" -- an apparent reference to Iran, which backs Hamas, and the militant group Hezbollah operating from southern Lebanon.

Biden was then due to deliver remarks at 1 pm (1700 GMT) from the White House.

The death toll in Israel has surged above 900 from the worst attack in the country's 75-year history, while Gaza officials have reported 765 people killed so far in Israeli air strikes.

Hamas has held around 150 hostages since its ground incursion, among them children, elderly and young people who were captured at a music festival where some 270 died.

- 'Barbarity' -

National Security Council spokesman John Kirby said the White House was closely watching the situation after Hamas warned it would start killing hostages every time Israel strikes a civilian target in Gaza without warning.

"You can't take a threat like that lightly," Kirby told CNN. "You've got to take it seriously because of the barbarity that Hamas has already shown that they're capable of."

The United States had offered Israel intelligence and "hostage recovery expertise," Kirby said.

He cautioned however that it was not yet confirmed that US citizens were among those held by Hamas, and that even if it was, it would be difficult to extract them from an active war zone.

Biden's response to the Israel-Hamas war is being closely watched at home, where Republicans accuse him of being soft on Iran, Hamas's sponsor, opening up a new front in the US election campaign.

But it is also being scrutinized in the region, as the US pledges its full support for Israel.

The US military at the weekend ordered its largest aircraft carrier to move closer to Israel in a show of support, along with other warships and a host of fighter jets. 

Kirby, the National Security Council spokesman, said on Monday that there was "no intention to put US boots on the ground" in the conflict between Israel and Hamas.

But he added that the "president will do what he has to do to look after our national security interests there and elsewhere."

Iran, which also backs Hezbollah, has denied any role in the attacks but looms large in the region.

dk/dw