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

As the Biden administration intensifies efforts to help American nationals leave Israel and Gaza, more than 20,000 U.S. citizens have reached out to the State Department asking for help in the wake of the Hamas attacks, including several hundred seeking to leave Gaza, according to two U.S. officials.

While not all have actively sought assistance with transportation, "many thousands" across Israel have indicated they needed help finding flights out of the country, the officials said.

Government-chartered planes have now started to ferry U.S. citizens from Israel to other countries in the region, though one official said the State Department still cannot share a detailed breakdown on the number of Americans whose departure it has already facilitated or the number of Americans still waiting for help "given this is an unfolding situation."

For Americans in Gaza, the U.S. is still eyeing the Rafah border crossing between Gaza and Egypt as a means of safe passage. Officials say they are very hopeful they will be able to work with Israel and Egypt to establish a corridor for U.S. citizens and other foreign nationals, but that it could take days to make substantial progress.

The Israeli government estimated that between 100 to 130 Americans were living in Gaza earlier this year. An official says the State Department believes hundreds more are visiting -- putting the total number in Gaza potentially between 500 and 600.

The State Department estimates that roughly 500,000 American citizens reside in Israel.

-ABC News' Shannon K. Crawford