Biden confirms US citizens are among Hamas hostages in sharp condemnation of attacks on Israel

President Joe Biden confirmed Tuesday that Americans are known to be among the hostages held by Hamas after its rampage over the weekend, delivering an emotional and angered denunciation of the terror and making clear he expected a forceful reprisal by Israel.

The president delivered vivid descriptions of the atrocities carried out by the militants, saying they had "butchered" and "massacred" innocents. Biden said the known number of Americans killed now stands at 14.

"It's abhorrent," he said. "The brutality of Hamas' bloodthirstiness brings to mind the worst rampages of ISIS."

The 10-minute speech appeared designed to ensure there is little daylight between the US and Israel at a moment of deep crisis. The president made no equivocation about the attacks and made no call for restraint by Israel as it responds, apart from insisting it follow the rule of law.

Recounting in stark terms the "sheer evil" of the attack, Biden warned other governments in the region not to step into the fray to exploit the situation: "I have one word: Don't."

Biden, who spoke earlier with Israeli Prime Minister Benjamin Netanyahu, offered his own view of what the country's response might look like.

"I told him that if the United States experienced what Israel is experiencing, our response would be swift, decisive, and overwhelming," Biden said from the State Dining Room.

The news that Americans are among the hostages taken by Hamas since the attack began on Saturday confirms what had largely been assumed by administration officials but had not been confirmed until Biden's speech.

Biden's national security adviser, Jake Sullivan, said later that the administration believes there are "20 or more Americans" believed to be missing from Israel Tuesday, but that that number does not necessarily reflect the number of hostages in Hamas custody.

Biden said during his speech that he was "directing my team to share intelligence and deploy additional experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts.

"As president, I have no higher priority than the safety of Americans being a hostage around the world," he said.

The president also said he intends to ask Congress to approve more funding for Israel to help it defend its territory and people. He promised his administration would not allow Israel to run out of ammunition and interceptors for its Iron Dome air defense system, which is intended to shoot rockets out of the air before they strike Israeli territory.

"This is not about party or politics, it's about the security of our world, the security of the United States of America," he said.

The remarks Tuesday, where he was flanked by Vice President Kamala Harris and Secretary of State Antony Blinken, were Biden's most substantive since the outbreak of violence early Saturday. Within hours of the deadly Hamas attack on Israel that left hundreds of people dead, the president was on the phone with Netanyahu. He has spoken with him three times since the violence began.

When Biden's counterpart - reeling from one of the worst attacks on his country in decades - has brought up the possibility of going into Gaza, the president has not offered warnings to him against doing so, a US official briefed on the conversation tells CNN.

That decision by the president to hold back from urging Netanyahu to exercise restraint in the immediate aftermath of the attacks in no small part reflects the sheer shock and breadth of Hamas' brutal attack on Israel that makes this moment different, officials say.

"We are not urging restraint right now," one official said.

In his remarks, Biden insisted any response must adhere to the rule of law, saying that is what differentiated Israel and the United States from the Hamas militants.

"Terrorists purposely target civilians, kill them. We uphold the laws of war. It matters. There's a difference," Biden said.

Later, Sullivan told reporters the two men "had the opportunity to talk through the difference between going full bore against Hamas terrorists and how we distinguish between terrorists and innocent civilians."

The president and his national security team are keenly aware of what appears to be a growing likelihood of Israel making a ground incursion into Gaza. Over the past 72 hours, officials have acknowledged what a deeply tenuous position that possibility puts the administration in. As a general matter, the US has historically urged for a ceasefire on all sides when conflicts have broken out in the region.

The Biden administration has usually been very intent on handling those kinds of discussions very privately, and officials said this time will be no different. Biden's first written statement over the weekend did not include the term "ceasefire" - an omission that struck current and former administration officials alike.

During his speech, Biden said the violence unleashed by Hamas was reminiscent of some of the worst attacks by ISIS in the previous decade.

"The brutality of Hamas, this blood-thirstiness, brings to mind the worst, the worst rampages of ISIS - this is terrorism, but sadly, for the Jewish people it's not new," Biden said. "This attack has brought to the surface painful memories and the scars left by millennia of anti-Semitism and genocide of the Jewish people, so in this moment, we must be crystal clear, we stand with Israel."

Domestically, he said, law enforcement officials "have stepped up security around centers of Jewish life," while "working closely with state local law enforcement and Jewish community partners to identify and disrupt any domestic threat that could emerge in connection to these horrific attacks."

This story has been updated with additional developments on Tuesday.

CORRECTION: An earlier version of this story incorrectly identified one of the individuals standing behind Biden during his speech. It was Secretary of State Antony Blinken.

CNN's Donald Judd contributed to this report.