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STORY: "This is an act of sheer evil."

President Joe Biden denounced Hamas and stressed U.S. support for Israel on Tuesday.

"We stand with Israel."

And he confirmed that American citizens were among those killed over the weekend.

"More than 1000 civilians slaughtered, not just killed, slaughtered, in Israel. Among them, at least 14 American citizens killed. [FLASH] This is terrorism, but sadly for the Jewish people, it's not new."

Hamas launched its surprise attack against Israel on Saturday.

Israel has struck back with air strikes, as the death toll from both sides continues to mount.

Biden outlined the U.S. military assistance being sent to help Israel in its fight, including ammunition.

"We're going to make sure that Israel does not run out of these critical assets."

He also voiced concern for Americans being held hostage by the Palestinian Islamist group.

"We now know that American citizens are among those being held by Hamas. I'm directing my team to share intelligence and deploy additional experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts."

Moments after Biden's speech, U.S. national security adviser Jake Sullivan said many Americans were still missing following the mass killings in Israel.

"We believe that there are 20 or more Americans who at this point are missing, but I want to underscore and stress that does not mean necessarily that there are 20 or more American hostages. Just that is the number who are currently unaccounted for."

Meanwhile, U.S. law enforcement agencies were taking steps to disrupt any domestic threat that may emerge.

BIDEN: "In cities across the United States of America, police departments have stepped up security around centers of Jewish life. [FLASH] There is no place for hate in America, not against Jews, not against Muslims, not against anybody."

The FBI said earlier it was "closely monitoring unfolding events" but added it "does not have specific and credible intelligence indicating a threat to the United States stemming from the Hamas attacks in Israel."