President Biden on Tuesday harshly condemned the “abhorrent” terrorist attacks over the weekend by Hamas as the death toll in the region climbed, vowing that the U.S. would fully support the Israeli people.

The president also confirmed that Americans are among those being held hostage, with officials saying as many as 20 Americans are unaccounted for.

Both Biden and his top national security adviser backed Israel’s right to respond with force, as Israeli forces prepared to go on the offensive, and signaled an upcoming push for Congress to provide additional support.

“In this moment we must be crystal clear: We stand with Israel,” Biden said in remarks from the State Dining Room. “And we will make sure Israel has what it needs to take care of its citizens, defend itself and respond to this attack. There’s no justification for terrorism. There’s no excuse.” 

Biden recounted the violent images coming out of Israel in recent days, referencing parents being killed as they tried to protect their children; young people being massacred while attending a music festival, babies being killed, women being raped, and grandparents being ripped from wheelchairs.

“It’s abhorrent,” Biden said. “The brutality of Hamas … brings to mind the worst rampages of ISIS. This is terrorism. But sadly, for the Jewish people, it’s not new.”

In addition to the Israeli casualties, hundreds of Palestinians have been killed in Gaza amid relentless bombings by Israel, which vowed retaliation for the unprecedented attack. More than a dozen American citizens have been killed, according to the White House.

Biden has spoken three separate times with Israeli Prime Minister Benjamin Netanyahu since Saturday, including a call Tuesday. Biden has met with his national security team each day since the crisis began Saturday, and the U.S. and European allies put out a statement Monday affirming support for Israel. Secretary of State Antony Blinken is set to travel to Israel this week.

The Biden administration has also repositioned military assets in the region to assist Israel and started sending munitions and interceptors to the Israelis to ensure their Iron Dome defense system has enough supplies. The administration is not currently planning to put U.S. forces on the ground, officials said Tuesday.

Biden confirmed Tuesday that American citizens are among those being held hostage by Hamas and said that the number of Americans killed in the terrorist attacks had risen to 14. 

National security adviser Jake Sullivan said that the U.S. doesn’t know the exact number of Americans being held hostage but that 20 or more Americans are currently unaccounted for in Israel. U.S. officials had previously warned that Americans were likely among those being held by Hamas.

Biden administration officials have consistently defended Israel’s response to the attacks by Hamas, including Tuesday.

The president in his remarks said Israel has a “duty to respond to these vicious attacks.” 

Sullivan also dismissed the suggestion that Israel’s response should be categorized as “retaliation” and instead was defending itself.

“This is about providing support to Israel as it seeks to defend its territory and deal with an ongoing imminent threat from Hamas terrorists who, as I said before, are acting a heck of a lot like ISIS terrorists in their barbarity and cruelty,” Sullivan said.

“I’m not here to draw red lines, or issue warnings, or give lectures to anybody,” he added in response to a question about whether Biden suggested that Netanyahu exercise any restraint.

The White House has stressed the personal nature of the attacks on Israel as well. National Security Council spokesperson John Kirby got choked up Monday discussing the images on the ground in a CNN interview, and the White House lit up in blue and white for the Israeli flag Monday evening, following a call from Sen. Lindsey Graham (R-S.C.) to do so.

Biden on Tuesday said he will ask Congress when it returns from recess “to take urgent action to fund the national security requirements of our critical partners.”

“This is not about party or politics. This is about the security of our world, security of the United States of America,” he said.

Biden is expected to package funding for Israel with additional funding for Ukraine’s war effort, which the White House has struggled to pass with an increasing number of House Republicans not supporting it.

Administration officials have also said they have existing appropriations and authority to support Israel without additional congressional action if needed.

Potentially complicating matters is that the House is without a Speaker after Kevin McCarthy (R-Calif.) was ousted from the role last week in a move led by a small number of Republicans. A minority of Republicans have expressed opposition to providing more aid to Ukraine, but support for Israel has faced no such resistance in the GOP-controlled House. 

Updated at 4:15 p.m. ET