US President Joe Biden has said that Israel has a duty and a right to respond to Hamas' "terrorism" as fighting continues near Gaza.
In his remarks, Mr Biden said that the US will "stand with Israel" and provide whatever assistance it needs.
Mr Biden said that at least 14 Americans have been killed in the fighting, with others taken as hostages.
He added there was no justification for the "rampages" by Hamas.
The identities of US citizens killed or missing have not yet been announced by officials.
In an approximately 10-minute speech at the White House, Mr Biden condemned what he termed "an act of sheer evil" from Hamas, comparing the attacks to "the worst rampages" of the so-called Islamic State.
"Like every nation in the world, Israel has the right to respond, and indeed has a duty to respond, to these vicious attacks," Mr Biden said, adding that the US response in a similar situation would be "swift, decisive and overwhelming".
Standing alongside Vice President Kamala Harris and Secretary of State Anthony Blinken, Mr Biden also said the US is sharing intelligence with Israel and has deployed experts to "consult with and advise Israeli counterparts on hostage recovery efforts".
"As president, I have no higher priority than the safety of Americans being held hostage around the world," he added.
At a separate news conference on Tuesday, National Security Advisor Jake Sullivan said that "20 or more" Americans are missing, although US officials are unsure of how many have been taken captive.
Mr Sullivan added that Mr Blinken - who has been focused on securing the release of hostages in Gaza - will travel to Israel "in the coming days" to meet with Israeli officials.
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.
Additionally, US defence officials have said they are working with their Israeli counterparts to expedite shipments of military supplies and ammunition to the Israel Defence Forces (IDF). Mr Biden said that he will ask Congress to take "urgent action" to fund the requirements of US allies.
"Let there be no doubt - the United States has Israel's back," Mr Biden said. "We'll make sure the Jewish and democratic state of Israel can defend itself today, tomorrow, as we always have".
In his remarks on Tuesday, Mr Biden said that the US stands "ready to move in additional assets as needed" - which he said is a message intended for other groups or countries in the region that may get involved.
"I have one word: don't," he said.
While the FBI has said it has no "specific intelligence" regarding threats in the US stemming from the conflict, Mr Biden said that police across the country have 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.
