At least 11 Americans have been killed as the Israel-Hamas conflict continues, according to President Joe Biden.

"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," he said in a statement on Monday. "Sadly, we now know that at least 11 American citizens were among those killed -- many of whom made a second home in Israel."

The statement continued, "It's heart-wrenching. These families have been torn apart by inexcusable hatred and violence...My heart goes out to every family impacted by the horrible events of the past few days."

It's currently unclear how many Americans are among those missing or might have been taken hostage, but Biden said in a statement on Monday that it's "likely" American citizens are being held hostage by Hamas.

Here's what we know about the U.S. victims so far:

Hayim Katsman, 32

The first American citizen identified is 32-year-old Hayim Katsman, who had been living in Israel, his mother told ABC News.

Hannah Katsman said she initially thought her son had been taken hostage, but later learned he had been killed when Hamas militants burst into his apartment.

She said he and a female neighbor were hiding in a closet when they were found. The neighbor was released but her son was shot dead and his body was found in his apartment, she said.

"[I've] been getting so many messages from people who worked with Hayim or who knew him, or who met him during their travels and how warm he was, how open," Hannah Katsman told ABC News. "He was a very accepting person and [a] very loyal friend, good sense of humor. He took things in stride."

According to the University of Washington's Henry M. Jackson School of International Studies, Hayim Katsman received his Ph.D. in 2021 with his research focusing on "the interrelations of religion and politics in the Middle-East, focusing on Israel/Palestine."