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 state-run Islamic Republic State TV announced Sunday that its broadcasting building was targeted in the joint U.S.-Israel attacks.
A state TV presenter said that an American-Israeli attack targeted the country's broadcasting building. The presenter added that state TV broadcasting continues as usual as technical teams assess the situation.
-ABC News' Somayeh Malekian
President Donald Trump claimed on Sunday that U.S. military forces "destroyed and sunk" nine Iranian naval ships and that Iran's naval headquarters had also sustained major damage since strikes against the country began on Saturday.
"I have just been informed that we have destroyed and sunk 9 Iranian Naval Ships, some of them relatively large and important," Trump said in a social media post. "We are going after the rest -- they will soon be floating at the bottom of the sea, also!"
The president said Iran's Naval Headquarters was also attacked, adding, "We largely destroyed their Naval Headquarters. Other than that, their Navy is doing very well!"
Earlier Sunday, U.S. Central Command said in an online post that "an Iranian Jamaran-class corvette was struck by U.S. forces during the start of Operation Epic Fury. The ship is currently sinking to the bottom of the Gulf of Oman at a Chah Bahar pier."
-ABC News' Nicholas Kerr and Cindy Smith
The CIA had been tracking Iran's Ayatollah Khamenei for months prior to Saturday's U.S.-Israeli attacks on Iran, a source familiar with the intelligence confirmed to ABC News.
When they learned there would be a meeting of top leaders on Saturday with the Supreme leader, the timing of the attack was moved, the source said, adding that the agency had high confidence in their analysis.
These details were first reported by The New York Times. Khamenei was killed in Saturday's attacks.
A second source told ABC News that Israel had been planning for months with its own intelligence to target Iranian leadership, according to a person familiar with the operation.
-ABC News' Cindy Smith and Anne Flaherty
Israeli Prime Minister Benjamin Netanyahu said Israeli forces were "striking the heart of Tehran with increasing strength, which will increase even more in the coming days."
Netanyahu's comments were part of a recorded video message in which said he had "given instructions for the continuation of the campaign" after meeting with senior Israeli officials, including "the minister of defense, the chief of staff and the head of the Mossad," Israel's national intelligence agency.
Netanyahu also said the joint U.S.-Israeli attacks had "eliminated dozens of senior officials of the oppressive regime" in addition to "the dictator Khamenei," referring to Iran's late Supreme Leader Ayatollah Ali Khamenei, who was killed in Saturday's attacks.
He also addressed reports of Israelis killed and injured in retaliatory strikes on Israel, saying "These are painful days. ... My heart goes out to the families."
"We are in a campaign in which we are bringing the entire strength of the IDF, as never before, to this campaign to ensure our existence and our future," Netanyahu said. "But we are also bringing to this campaign the assistance of the United States, my friend, the President of the United States, Donald Trump, the U.S. military. This combination of forces allows us to do what I have been hoping to do for 40 years: to strike the terrorist regime squarely in the face. I promised, and so we will."
-ABC News' Victoria Beaule
