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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Iran's Guardian Council announced on Sunday that it had selected Ayatollah Alireza Arafi to join the three-person temporary leadership council formed to lead Iran following the killing of Supreme Leader Ali Khamenei.
Arafi will join President Mahmoud Pezeshkian and the head of the Iranian judiciary on the council.
-ABC News' Somayeh Malekian and Victoria Beaule
The Israel Defense Forces said Sunday it had killed 40 Iranian commanders in strikes on Iran.
Those killed, the IDF said, included the chief of staff of the Iranian armed forces, "seven members of the top Iranian security leadership" and "the majority of the highest-ranking senior military officials."
-ABC News' Jordana Miller and Victoria Beaule
Bahrain's National Communication Center said on Sunday morning that it was been targeted by another wave of "Iranian ballistic missiles and drones."
Air defense systems "responded with high efficiency and full operational readiness," the statement said, repotting "limited debris" across several areas.
The statement did not say how many missiles and drones were involved in the latest wave of attacks. As of Saturday night, Bahraini authorities said the country had intercepted 45 missiles and nine drones.
Hundreds of protesters stormed the U.S. consulate in Karachi -- Pakistan's largest city on the country's southern coast -- on Sunday to protest the killing of Iranian Supreme Leader Ayatollah Ali Khamenei.
At least six protesters were killed in the clashes, local authorities said. Police used tear gas to disperse the demonstration.
In a post to X, the U.S. Embassy in the capital Islamabad said it was "monitoring reports of ongoing demonstrations at the U.S. Consulates General in Karachi and Lahore, as well as calls for additional demonstrations at U.S. Embassy Islamabad and Consulate General Peshawar."
"We advise U.S. citizens in Pakistan to monitor local news and observe good personal security practices, including being aware of your surroundings, avoiding large crowds and ensuring your STEP registration is up to date," it added.
-ABC News' Habibullah Khan and Ellie Kaufman
