WASHINGTON (CNN) — Eleven U.S. citizens have died in the conflict in Israel, President Joe Biden said Monday, and an unknown number remain missing.

“As we continue to account for the horrors of the appalling terrorist assault against Israel this weekend and the hundreds of innocent civilians who were murdered, we are seeing the immense scale and reach of this tragedy,” Biden said in a statement. “Sadly, we now know that at least 11 American citizens were among those killed — many of whom made a second home in Israel.”

It is “likely,” Biden said, that American citizens may be among those being held hostage by Hamas, and that his administration is working with Israeli officials on “every aspect of the hostage crisis.”

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Biden also noted that there are American citizens whose whereabouts remain unaccounted for.

“This is not some distant tragedy. The ties between Israel and the United States run deep,” he said. “It is personal for so many American families who are feeling the pain of this attack as well as the scars inflicted through millennia of antisemitism and persecution of Jewish people.”

State Department spokesman Matt Miller told CNN’s Phil Mattingly on Monday that U.S. authorities are in close contact with Israel’s government and the families of those affected by the attack.

U.S. authorities have been scrambling to establish how many Americans have been killed or taken hostage in the conflict. Secretary of State Antony Blinken told CNN’s Dana Bash on “State of the Union” Sunday that the US was “working overtime” to verify reports of missing and dead Americans, and Israeli Minister of Strategic Affairs Ron Dermer said Americans are among the “scores” of hostages being held in Gaza.

Qatar has been in talks with Hamas about the hostages the terror group is holding inside Gaza, and the US has been coordinating with the Qataris as they play a key mediating role with Hamas, a senior US official and another person familiar with the discussions told CNN.

US officials at the White House and State Department, including Blinken and Assistant Secretary of State for Near Eastern Affairs Barbara Leaf, have remained in touch with the Qataris throughout the weekend as they communicate with Hamas. CNN has reached out to the governments of Qatar and Israel for comment.

The US has also pledged to provide additional military support in the coming days, though domestic political dysfunction could hamper the response.

Secretary of Defense Lloyd Austin announced on Sunday that he has ordered the US Navy’s Ford carrier strike group to the eastern Mediterranean, near Israel. The USS Gerald Ford is the Navy’s most advanced aircraft carrier and it is being deployed to the area, along with a guided missile cruiser and four destroyers, as a deterrence measure, Austin said.

But the current commander of the US Navy’s 5th fleet, which is responsible for US naval operations in the Middle East region including the Red Sea and Gulf of Oman, is still awaiting promotion to deputy commander of US Central Command, which oversees US forces and operations in the region following a hold by Republican Sen. Tommy Tuberville over military confirmations.

Meanwhile, as the Biden administration looks to provide additional assistance to Israel, officials were unsure over the weekend about what could be accomplished without a sitting speaker. Acting Speaker Patrick McHenry has little power outside of recessing, adjourning or recognizing speaker nominations, and it’s unclear whether he can participate in intelligence briefings on the crisis.

Administration officials said they will look to the current $100 million in Presidential Drawdown Authority, which allows the rapid dispatch of weapons from existing stocks, to send more aid immediately, according to a person familiar with the discussion. The drawdown will likely need additional funds from Congress, the officials told lawmakers.