As Iran retaliates, largest US military base in Middle East hit by ballistic missile, Qatar says

Multiple Israeli officials tell ABC News that Israel has determined that Iran's Supreme Leader Ayatollah Ali Khamenei was killed in the joint U.S. and Israeli strikes on Tehran earlier Saturday. U.S. officials have been informed by the Israelis but have not independently confirmed the report, according to two U.S. officials.

President Donald Trump told ABC News he believes the supreme leader is dead, saying, "we believe so."

"I don't want to say anything definitively until I see things but we believe he is. And much of their leaders are gone," Trump told ABC News.

Earlier Saturday, an Iranian Foreign Ministry Spokesperson Esmaeil Baghaei denied reports that Khamenei and Iranian President Masoud Pezeshkian were killed in the strikes, saying they are "safe and sound," in an interview on ABC News Live.

The Israel Defense Forces said it believes almost the entirety of Iran’s military leadership was killed Saturday, including the head of the Iranian Security Council, the defense minister and a top intelligence official.

Among the officials are Ali Shamkhani, the secretary of the Iranian Security Council; Mohammad Pakpour, commander of the Islamic Revolutionary Guard Corps; and Aziz Nasirzadeh, the defense minister of Iran.

-ABC News' Jordana Miller, Dana Savid, John Santucci, Anne Flaherty, Shannon Kingston and Luke Barr

President Donald Trump expressed optimism about the U.S.-Israeli attack on Iran saying it is going "very well," speaking to ABC News by phone.

Asked about how long it will last, Trump said, "As long as we wanted to, actually. But it’s done such damage already. It’s like — they are incapacitated, essentially -- as long as we wanted to."

Asked about Iranian leadership and whether the U.S. believes they were killed in strikes, Trump said, "A lot of it is, yeah. But we don’t know all, but a lot of it is. Was a very powerful strike."

Asked about the next leadership and if a leader has been identified, Trump said "Yes. We have a very good idea."

-ABC News' Jonathan Karl

U.S. Department of State Consular Affairs is urging Americans around the world, especially in the Middle East, to "exercise increased caution" and "follow the guidance in the latest security alerts issued by the nearest U.S. embassy or consulate."

Travelers "may experience travel disruptions due to periodic airspace closures," the department said.

Travelers were also encouraged to join the Smart Traveler Enrollment Program (STEP) "to receive the latest security alerts and follow the 'U.S. Department of State - Security Updates for U.S. Citizens' channel on WhatsApp" to receive travel alerts and updates, the department urged.

The White House is working to arrange full Senate and House member briefings this week on the Iran strikes, according to an administration official.

-ABC News' Selina Wang