President Biden on Monday announced that the number of Americans killed in terrorist attacks against Israel had risen to 11, and he warned that American citizens may be among those being held hostage by the militant group Hamas.

“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.”

U.S. officials are still working to confirm whether Americans are among those being held by Hamas, but said “it is likely” that is the case. “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,” Biden said.

John Kirby, a White House spokesperson on national security issues, told reporters on a call later Monday that the administration can’t confirm Hamas is holding Americans hostage, but that there are still many Americans who are unaccounted for.

“So I can’t say definitely that we know Americans are being held hostage,” Kirby said. “That said, we have to accept the grim possibility that some are.”

The State Department is providing consular assistance to Americans who are currently in Israel and urged those in the area to follow the guidance of local authorities.

More than 1,000 people have died since Hamas, a militant group that controls Gaza, launched attacks against Israel, triggering a response by Israeli forces. Prime Minister Benjamin Netanyahu said Saturday that Israel is “at war” with Hamas following the initial attacks.

Biden has spoken multiple times with Netanyahu since the attacks first unfolded, and he met earlier Monday with his national security team. The president has vowed to assist Israel as it deals with the fallout of the attacks, and the U.S. has already repositioned military assets in the region and sent some munitions to Israel.

In Monday’s statement, Biden called the U.S. and Israel “inseparable partners.”

“This is not some distant tragedy. 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,” Biden said in Monday’s statement.

“In cities across the country, police departments have 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 horrific terrorist attacks in Israel.”

Updated at 6:09 p.m.