(NewsNation) — As Hamas continues to cause terror in Israel, President Joe Biden has confirmed that at least 11 Americans were killed, and it’s “likely” that more U.S. citizens are among the hostages in Gaza. However, no official confirmation has been made yet.

John Kirby, the White House National Security Council spokesperson, emphasized Monday night on NewsNation that the safety of Americans overseas remains Biden’s top concern.

“It’s deeply alarming and deeply concerning, and we have to take it seriously. It’s another example of the barbarity of this group. And the indiscriminate violence that they’re willing to visit upon innocent civilians. We call on Hamas to immediately release every single hostage that they’re holding,” Kirby said. “And obviously, we’re going to continue to work with the Israeli officials to do what we can to gain information about these hostages, and certainly assist in any way that might be appropriate in trying to get those hostages home. Nothing’s more important to President Biden than the safety and security of Americans overseas. And this is this case, certainly is no exception to that.”

This situation comes with a warning from Hamas. It’s stated that if Israel were to bomb civilians in Gaza without warning, it would publicly execute hostages.

Israel is currently mobilizing 300,000 reservists, a move necessitated by the escalating situation. In a video, injured soldiers were seen being rescued by Israel Border Police This increase in troop deployment is boosting speculation that Prime Minister Benjamin Netanyahu may be planning a vast ground offensive in Gaza to secure the hostages.

Hamas, along with other militant groups in Gaza, claims it’s holding more than 130 soldiers and civilians who were taken from inside Israel over the weekend.

Many of these kidnappings have been captured on cellphone cameras. The White House expressed its concern Tuesday morning about several Americans who are unaccounted for. Some of them may have lost their lives, while others could be among the hostages.

Although the details are not yet clear, the United States is committed to collaborating with Israel to rescue any Americans who might have been held by Hamas in the face of this execution threat.

The White House has also confirmed that some of the military aid, specifically the initial shipment of security systems, is currently en route to Israel.

Biden is scheduled to address the nation about the conflict Tuesday at the White House.