US President Joe Biden has said Israel has a right and a duty to respond to Hamas’ attacks over the weekend, which he called an “act of sheer evil”.
In forceful remarks, he said the US would “stand with Israel” and provide whatever assistance it needed.
Mr Biden said at least 14 Americans had been killed in the attacks and other US citizens had been taken hostage.
In a later briefing on Tuesday, a national security spokesman said at least 20 Americans were missing.
It is not clear how many of those have been taken captive by Hamas, National Security Advisor Jake Sullivan said.
Militants from the group, which is designated a terrorist organisation by the US, breached the border fence in the Gaza Strip in several places on Saturday and launched the most serious cross-border attack Israel has faced in a generation.
Some 1,200 people have been killed in Israel since Saturday. More than 950 people have died in retaliatory Israeli air strikes on Gaza.
Standing alongside Vice President Kamala Harris and Secretary of State Antony Blinken at the White House, Mr Biden said there was no justification for the “rampages” by Hamas militants.
“There are moments in this life, and I mean this literally, when the pure unadulterated evil is unleashed on this world,” Mr Biden said. “The people of Israel lived through one such moment this weekend.”
He condemned what he called “stomach turning” reports of “parents butchered” and “entire families slain”.
“We will work hour-by-hour both to determine whether we can account for any of those Americans, or to confirm exactly the number of Americans held hostage,” he added.
The US is sharing intelligence with Israel, Mr Biden said, and had deployed experts to “advise Israeli counterparts on hostage recovery efforts”.
Earlier this week, the US announced plans to move naval forces – including an aircraft carrier – to the eastern Mediterranean. US Air Force squadrons based in the region are also being bolstered with additional combat aircraft.
Mr Biden said he would ask Congress to take urgent action to provide more support. The White House is considering submitting a request to Congress that would include more military aid for both Ukraine and Israel, a source told Reuters.
Mr Biden and Ms Harris spoke with Israeli Prime Minister Benjamin Netanyahu on Tuesday and pledged to supply fresh ammunition for Israel’s Iron Dome missile defence system.
Mr Blinken is expected to travel to Israel on Wednesday to meet senior Israeli officials.
Israel has massed hundreds of thousands of troops as well as tanks and other heavy armour around Gaza, prompting speculation it is readying for a potential ground invasion as air strikes continue.
While the FBI has said it has no specific intelligence regarding threats in the US stemming from the conflict, Mr Biden said police across the country had stepped up security around Jewish sites “to identify and disrupt any domestic threat that could emerge in connection with these horrific attacks”.
“There is no place for hate in America. Not against Jews, not against Muslims, not against anybody,” he said.