WASHINGTON U.S. President Joe Biden said on Monday that at least 11 American citizens were among those killed in Israel following the weekend's attacks by Palestinian Islamist group Hamas.

Washington believes it is likely U.S. citizens are also among those being held hostage by Hamas, he said in a statement.

Biden said the United States was working with Israeli officials to obtain more information as to the whereabouts of U.S. citizens who are still unaccounted for.

"For American citizens who are currently in Israel, the State Department is providing consular assistance as well as updated security alerts. For those who desire to leave, commercial flights and ground options are still available," he said, adding that they should take precautions and follow the guidance of local authorities.

Hamas gunmen burst across the fence from Gaza on Saturday, killing Israeli soldiers and civilians and taking dozens of hostages back to the coastal enclave. Israel responded with its heaviest bombardment of Gaza and could be contemplating a ground assault on the territory it withdrew from nearly two decades ago after 38 years of occupation.

Biden said he has directed his team to work with Israeli counterparts on every aspect of the hostage situation, including sharing intelligence and deploying experts from the U.S. government.

Police departments across the U.S. have stepped up security around centers of Jewish life, he added.

(Reporting by Jasper Ward and Kanishka Singh; editing by Dan Whitcomb and Rosalba O'Brien)