At least four American citizens were killed in the attacks in Israel over the weekend, senior administration officials told top House lawmakers on a call Sunday evening, according to multiple sources familiar with the matter.

That figure could rise in the coming days, the Biden administration officials told Congress. The administration is also still investigating unconfirmed reports of American citizens being taken hostage by Hamas.

Participants on the call included senior members of the relevant House committees and party leaders — including Rep. Patrick McHenry, R-North Carolina, the temporary House speaker, two sources told ABC News.

As previously reported, Senate leaders will receive a similar briefing tonight.

--ABC News' Ben Siegel

Senate leadership, chairs and ranking members of relevant committees will receive an unclassified briefing on the situation in Israel Sunday night at 9 p.m. ET, a Senate source told ABC News.

The chairs and ranking members from the Senate Committee on Foreign Relations, the Senate Select Committee on Intelligence and the Senate Committee on Armed Services are among those who will be briefed.

The president of Iran -- Ebrahim Raisi -- called the attack by Hamas on Israel as a “victory,” Reuters reported on Sunday.

At a cabinet meeting, he referred to the attack as a “manifestation of resistance,” the outlet also reported.

American citizens were killed in the attacks on Israel, U.S. officials confirmed Sunday.

"We can confirm the deaths of several U.S. citizens. We extend our deepest condolences to the victims and to the families of all those affected," a U.S. official told ABC News in a statement.

--ABC News' MaryAlice Parks, Shannon Crawford

In his first on-camera comments about Israel’s evacuation request for northern Gaza, Israeli Defense Minister Yoav Gallant urged Gaza residents to head south. He noted that Hamas will use them as "camouflage," “therefore we need to separate them."

"Those who want to save their life, please go south. We are going to destroy Hamas infrastructures, Hamas headquarters, Hamas military," he said.

Defense Secretary Lloyd Austin, who appeared at the press conference with Gallant, dodged ABC News' question about how Israel could end the Hamas terror threat without inevitably risking civilian lives. Austin called Israel's military "professional" and "disciplined."

Austin described how ISIS had also embedded themselves in civilian populations and how the anti-ISIS coalition "protected civilians and created corridors for humanitarian movement even in the midst of a pretty significant fight. So again, this is a professional force. It is well led, and I have every expectation that it will be disciplined."

Austin added, "I would tell you that in countering ISIS, I felt as if we were staring evil in the eye, it was truly evil. And what we've seen from Hamas, it takes that evil to another level."

-ABC News' Matt Seyler and Luis Martinez