At least 11 U.S. citizens were killed in Israel, Biden says, and others may be captives of Hamas.

At least 11 U.S. citizens have been killed in Israel and an unknown number are still unaccounted for, President Biden said in a statement on Monday evening, saying that “we believe it is likely that American citizens may be among those being held by Hamas.”

“I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts,” Mr. Biden said, while expressing anguish over the suffering of Israelis from “inexcusable hatred and violence.”

“This is not some distant tragedy,” Mr. Biden said in the statement. “The ties between Israel and the United States run deep. 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.”

He noted that police departments across the United States had “stepped up security around centers of Jewish life, and the Department of Homeland Security, the Federal Bureau of Investigation, and other federal law enforcement partners are closely monitoring for any domestic threats” in connection with the attacks in Israel.

A number of Americans who had been visiting Israel during the attack, including two lawmakers — Senator Cory Booker of New Jersey and Representative Dan Goldman of New York — were able to leave the country over the weekend. Mr. Biden directed Americans trying to leave Israel to seek out remaining “commercial flights and ground options” and urged all American citizens in Israel to “please also take sensible precautions in the days ahead and follow the guidance of local authorities.”

Mr. Biden spoke with the Israeli prime minister, Benjamin Netanyahu, in a call on Sunday, in which they discussed the hostages taken by Hamas, as well as what military assistance the United States could provide. The Pentagon announced the same day that it was sending additional munitions to Israel and moving more Navy warships, including an aircraft carrier, and combat aircraft closer to Israel in a show of support. Secretary of State Antony J. Blinken said the United States was working to fulfill several specific requests from Israel for military assistance, without providing details.

On Monday, Defense Department officials said the Pentagon had offered the assistance of U.S. Special Operations forces for planning and surveillance in any hostage recovery efforts. A White House spokesman also clarified that the U.S. government was not sure that American were being held. “We can’t confirm that they are, in fact, holding hostages,” John F. Kirby, the spokesman, told reporters. There are, he said, Americans who are unaccounted for and may be captive.

In the statement on Monday, Mr. Biden evoked the Sept. 11, 2001, attacks on the United States, lamenting the deaths of more Americans in an extremist attack.

“We remember the pain of being attacked by terrorists at home, and Americans across the country stand united against these evil acts that have once more claimed innocent American lives,” Mr. Biden said. “It is an outrage.”

The White House will be lit up in the blue and white colors of the Israeli flag on Monday night to express solidarity with Israel. It will be the latest landmark around the world to be so illuminated, joining the Empire State Building in New York, the Brandenburg Gate in Berlin and the Sydney Opera House in Australia, among others.

Eric Schmitt and Peter Baker contributed reporting.