President Joe Biden said on Monday that at least 11 American citizens are confirmed to be among the massive death toll resulting from an attack this weekend by Hamas fighters and Israel’s continued retaliatory bombardment on Gaza.

In a statement from the White House, Biden also said that there are still American citizens that remain unaccounted for, and that the United States government is working with Israeli officials to obtain more information on their whereabouts.

“My heart goes out to every family impacted by the horrible events of the past few days. The pain these families have endured, the enormity of their loss and the agony of those still awaiting information is unfathomable,” the president said.

After Saturday’s massive attack by Hamas, Israel formally declared war on the armed militants that rule the 2.3 million Palestinians who live under occupation in struggling Gaza. The Israeli military has since increased airstrikes on Gaza and threatened a potential ground invasion, while sealing the territory off from food, fuel and supplies.

Around 900 people have been killed in Israel as of Monday evening, according to local media. More than 680 have been killed in Gaza, according to Palestinian authorities. Thousands have been wounded on both sides, and more than 123,000 Palestinians have fled their homes in Gaza.

Hamas has claimed that it is holding about 100 hostages captive as part of its attack on Israel. In response to the Israeli airstrikes, a spokesman for Hamas’ armed wing said that the group will kill one Israeli civilian captive any time Israel targets Palestinian civilians in their homes in Gaza “without prior warning.”

Biden said that the U.S. government believes it is “likely” that American citizens are among the Hamas captives.

“The safety of American citizens — whether at home or abroad — is my top priority as President,” the president’s statement read, adding that he has directed his 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.”

The State Department is currently providing consular assistance and updated security alerts for American citizens who are still in Israel. Americans who choose to leave Israel can still do so via commercial flights and other ground options. The president warned Americans still in Israel to “take sensible precautions” and follow local authorities’ guidance.

“In this moment of heartbreak, the American people stand shoulder-to-shoulder with Israelis. 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,” Biden said.

“It is an outrage. And we will continue to show the world that the American people are unwavering in our resolve to oppose terrorism in all forms.”

Biden’s statement did not include anything about the Palestinian civilians killed by Israeli airstrikes, the Gazans now displaced, or the push by far-right Israeli officials to block Palestinians’ access to food, water and electricity. It also did not mention the root cause — unending apartheid carried out daily by Israel against Palestinians — that has led to the present moment.

Since the attack this weekend, the White House has been working diligently to get a Senate confirmation hearing for Biden’s nominee for the U.S. ambassador to Israel, according to NBC News. The Senate Foreign Relations Committee is reportedly expected to hold a confirmation hearing as early as Oct. 18 for former Treasury Secretary Jack Lew.