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

The United States is warning against other countries from becoming involved in the Israel-Gaza conflict amid fears that it could spread into a wider war in the Middle East.

"This is not the moment for other parties who are hostile to Israel to seek advantage or to seek to exploit the attacks that have taken place," Jon Finer, deputy national security adviser to U.S. President Joe Biden, Monday on "Good Morning America."

"That is part of why the United States has moved the Carrier Strike Group to the eastern Mediterranean to send a strong and unmistakeable signal that no one else should get involved in this," he added. "We'll see how things unfold in the course of the coming days."

Finer also emphasized that the U.S. believes "Israel has every right to defend itself full-stop."

"Israel will ultimately make the decisions about how it chooses to go about and conduct that defense," he said. "We are offering support in a number of ways."

Finer noted that "more U.S. steps to show support and solidarity for Israel" can be expected.

"The U.S. is going to offer support at every level for Israel's defense and we are working through details those details with our intelligence professionals, our military and our diplomats," he added. "We are in daily -- in fact, many times a day -- contact with Israeli counterparts to see what they need and offer it."

The U.S. "strongly suspect[s] that there will be American citizens among those killed" in the ongoing conflict, according to Finer.

"We are looking obviously very intensively into whether there were any Americans were among those who have been abducted and brought to Gaza. This is still ongoing," he said. "There is still fighting inside Israel as we speak and there is a bit of fog of war in terms of the ability to gather specific information. And so, we will have much more to say about this at the right time."

When asked for comment on a recent report by The Wall Street Journal that Iran was behind Hamas' latest attack on Israel, Finer responded: "We have no direct information to confirm that report. We've obviously seen it, we're looking into it, but we do not have the ability to corroborate it at this time."

"What we can be quite clear about is that Iran is broadly complicit in these attacks for having supported Hamas going back decades -- for having provided financial support, for having provided training, for having provided weapons to Hamas," he added. "What we don't have is direct information that shows Iranian involvement in ordering or planning the attacks that took place over the last couple days. It's something that we’re going to keep looking at closely."