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

Human Rights Watch has accused Israel of using white phosphorus munitions in military operations in Gaza on Wednesday, saying it violated international humanitarian law by putting civilians at unnecessary risk.

In a report released Thursday, Human Rights Watch said it based its determination on a video it said it has verified, as well as two eyewitness accounts.

The Israel Defense Forces said in a statement to ABC News that claims they used white phosphorus in Gaza are "unequivocally false."

ABC News geolocated a video showing a jellyfish-shaped plume of white smoke over the port of Gaza. The video was posted on Wednesday by the official Palestinian news agency WAFA and matches images posted on social media and by other news outlets on the day.

Two experts -- Amael Kotlarski, the weapons team manager for the defense intelligence firm Janes, and Amnesty International weapons investigator Brian Castner -- told ABC News that images suggested munitions deployed over the port of Gaza on Wednesday appeared to contain white phosphorus.

"We have verified that Israeli artillery forces striking Gaza are equipped with M825 and M825A1155mm white phosphorus projectiles," Castner told ABC News, pointing to photographs by an Anadolu Agency photographer taken on Monday and showing Israeli troops handling shells labeled "M825A1."

Kotlarski said visual evidence alone was not sufficient to confirm that these specific shells were used in the Gaza port strikes.

There are "perfectly legal" military uses for white phosphorus munitions, Kotlarski said, including to create a smoke screen to conceal movements, to mark targets for artillery or air strikes, or to illuminate a battlefield. However, its use in urban areas is "problematic due to the higher risk of collateral damage," Kotlarski said, including the munitions' potential to cause severe burns.

White House Principal Deputy Press Secretary Olivia Dalton said Friday the U.S. had not independently verified allegations Israel used white phosphorous "so that is not something I would want to speculate about or weigh in on at this time."

-ABC News' Christopher Looft, Ben Gittleson and Jordana Miller