Hezbollah's leadership 'nearly decimated,' national security spokesperson Kirby says

White House National Security spokesman John Kirby said on ABC News' "This Week" that Hezbollah's command structure has been "nearly decimated" and that thousands of the terrorist group's drones and missiles have been destroyed.

Kirby's remarks come after the Israel Defense Forces confirmed on Saturday that it had killed Hezbollah Secretary General Hassan Nasrallah in a strike on Beirut on Friday.

"There's no question that the Hezbollah of today is not the Hezbollah that was even just a week ago," Kirby told ABC's Martha Raddatz.

But Hezbollah will likely regroup, Kirby warned, saying that "somebody will be anointed" to take Nasrallah's place in command of the organization.

When asked about what capabilities remain for Hezbollah -- whether the group still has long-range missiles and other firepower -- Kirby said the U.S. does not have a "clear picture" of their capabilities.

"You can't just count them out," Kirby said. "They still have quite a bit of capability left."

Kirby also warned that the U.S. has to "be ready" for Iran to respond to the Israeli attack that killed Nasrallah and other leaders, saying that Iran's rhetoric "certainly suggests they're going to try to do something."

"We're watching this very, very closely to see how -- if and how -- Hezbollah and or Iran may react as well as the militia groups in Iraq and Syria. We have to be prepared for some sort of response. We have to make sure that we are ready, and we are. We believe we have the force capability we need in the region, but it's not clear right now, it's too soon to know how Iran's going to react to this," Kirby said.

Kirby's comments come as the U.S. beefs up military posture in the region. Earlier this week, the Pentagon confirmed that it moved a "small number" of additional U.S. military personnel to the Middle East.

President Joe Biden said in a statement on Saturday that he directed the Department of Defense "to further enhance the defense posture of U.S. military forces in the Middle East region to deter aggression and reduce the risk of a broader regional war." Raddatz pressed Kirby on what the president meant by that.

"We did deploy some additional forces into the region. I would tell you that there's other options available as well in terms of adding and enhancing that force posture," Kirby said.

Kirby added that the U.S. has been growing its capacity in the region, even more than when Iran launched a missile and drone attack on Israel in April.

"So there is already a very robust military capability to defend ourselves and to help defend Israel, if it comes to that," Kirby said.

Raddatz also asked Kirby about whether the administration was having conversations with Israel about how their actions could lead to U.S. assets becoming targets. Kirby said that the Israelis "understand" that the U.S. could become targets of attacks.

"I think they understand that, and we're having those discussions with them right now about sort of what is the next best step, or steps forward, at the same time, as the president said yesterday, we got to make sure we've got the ability to defend our troops and our facilities, if it comes to that, and we believe we do," Kirby said.

ABC News' Luis Martinez contributed to this report.