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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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.
The following three officials will act collectively as a temporary leadership council to govern Iran, Iranian state media reported.
The three individuals who will form the council are the Iranian President Masoud Pezeshkian, the Iranian Judiciary Chief Gholamhossein Mohseni Ejei, and a jurist from the Guardian Council, Iranian state media said.
A fire continues to burn at Erbil international airport in the Kurdistan region of Iraq after an explosion at daybreak. The Iranian-backed group Islamic Resistance in Iraq has now claimed responsibility for the strike.
What exactly is on fire remains unclear, but it is burning in an area that is comprises a number of military base buildings.
This base has been the target of repeated missile and drone attacks in the past.
-ABC News' Bader Katy and Paul P. Murphy
Iranian state TV confirmed Iranian Supreme Leader Ayatollah Ali Khamenei's death Saturday night.
Khamenei was "carrying out his assigned duties," and was "in his workplace" when he was killed "in the early hours of Sunday morning," according to the announcement.
Iranian state TV said claims from Israeli media that Khamenei was "in a secure and hidden location" when he was killed were false.
-ABC News' Ellie Kaufman
