Sgt. Benjamin N. Pennington, 26, was killed in Saudi Arabia.
President Donald Trump announced "major combat operations" against Iran on Feb. 28, with massive joint U.S.-Israel strikes attack targeting military and government sites, officials said.
Iranian state television confirmed that Ayatollah Ali Khamenei was among those killed in Tehran on the first day of strikes. His son Mojtaba Khamenei was chosen on Sunday to succeed him.
Iran is responding to the operation with missile and drone attacks targeting Israel, regional U.S. bases and multiple Gulf nations. Israel is also intensifying its long-running strike campaign against the Iran-backed Hezbollah militia in Lebanon.
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Ayatollah Ali Khamenei was killed during the "opening blow" of the U.S.-Israel operation on Saturday, Israeli Defense Minister Israel Katz said in a post on X on Sunday.
Katz congratulated Israeli Prime Minister Benjamin Netanyahu for his "leadership and determination," and the Israel Defense Forces for its "brilliant execution."
-ABC News' Jordana Miller and Ellie Kaufman
Iranian state media confirmed the deaths of Rear Adm. Ali Shamkhani, the secretary of the Defense Council, and Maj. Gen. Mohammad Pakpour, the commander-in-chief of the Islamic Revolutionary Guards Corps, in strikes on Saturday.
ABC News' Somayeh Malekian and Ellie Kaufman
In a new post on his social media platform on Sunday morning, President Donald Trump warned Iran not to escalate its attacks, following U.S. strikes against the country.
"Iran just stated that they are going to hit very hard today, harder than they have ever hit before," Trump wrote in the post. "THEY BETTER NOT DO THAT, HOWEVER, BECAUSE IF THEY DO, WE WILL HIT THEM WITH A FORCE THAT HAS NEVER BEEN SEEN BEFORE!"
The Iranian Revolutionary Guard Corps took responsibility for targeting 27 U.S. bases in the Middle East region in a statement broadcast by Iranian state media Sunday morning.
Sirens were sounding in Bahrain at about 8 a.m. local time, Bahraini state TV confirmed.
In addition to the bases, the IRGC statement said it was targeting the large defense industrial complex in Tel Aviv.
