President Joe Biden says the weekend attack by Hamas militants on Israel was the deadliest against Jews since the Holocaust.
The president said: “This attack was a campaign of pure cruelty, not just hate, but pure cruelty against the Jewish people.”
Mr Biden made the remarks while speaking with Jewish leaders on Wednesday as the US said the number of its citizens killed had risen to 22.
It is an increase to the death toll of 14 the day before.
The war has already claimed more than 2,200 lives on both sides.
John Kirby, the White House national security spokesman said a “handful” of US citizens are among the estimated 150 hostages captured by Hamas militants during their shocking weekend assault on Israel.
In a further sign of US support for Israel, Secretary of State Antony Blinken left for meetings with officials there and the US military is moving a second aircraft carrier towards the Mediterranean Sea as part of efforts to prevent the war from spilling over into a more dangerous regional conflict.
Mr Kirby said the USS Dwight D Eisenhower and its ships would be an “available asset” if necessary.
The USS Gerald R Ford, the Navy’s most advanced aircraft carrier, and its strike group have already arrived in the Eastern Mediterranean.
The attack has raised questions about the role of Iran, the main sponsor of Hamas, and whether it was directly involved in the operation, but the US has collected information that suggests senior Iranian government officials were caught off guard by the multipronged assault.
That piece of intelligence has informed White House officials publicly asserting that it has not yet seen evidence of direct involvement by Iranians in the planning or execution of the Hamas attack.
“We haven’t seen anything that tells (us) they have specifically cut checks to support this set of attacks, or that they were involved in the training. And obviously, this required quite a bit of training by these terrorists,” Kirby said, though he added that the US will continue to look at the intelligence “and see if that leads us to a different conclusion”.
With many airlines suspending commercial flights in and out of Israel because of the ongoing rocket and missile exchanges, Mr Kirby said the United States was exploring “a range of other options” to assist Americans who want to leave, appearing to leave open the possibility of a US-assisted evacuation.
He said the administration was still in talks with Israel and Egypt to try to arrange safe passage for Gaza’s civilians.
“These people are victims, too,” he said. “They didn’t ask Hamas to do this.”
President Joe Biden, who is set to meet with Jewish leaders later on Wednesday, sought to connect the Hamas attacks directly to decades of antisemitism and violence endured by Jews around the world.
“This attack has brought to the surface the painful memories and scars left by a millennium of antisemitism and genocide against the Jewish people,” Mr Biden told reporters.
He added: “We have to be crystal clear. There is no justification for terrorism, no excuse and the type of terrorism that was exhibited here is just beyond the pale, beyond the pale.”
Mr Biden said he and Vice President Kamala Harris spoke by phone on Wednesday with Mr Netanyahu.
It was at least the fourth call between Mr Biden and Mr Netanyahu since Saturday’s attack.
“The United States has Israel’s back and we’re going to be working on this through the day and beyond,” Mr Biden said.