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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Israel and the U.S. have launched a joint attack on Iran, according to two U.S. officials.
The attacks have begun and they are being described as not small-scale, according to one U.S. official.
The U.S. embassy in Qatar has told Americans to shelter in place, saying "all embassy personnel [are] directed to shelter in place and all Americans in the country are encouraged to do the same."
"The U.S. Embassy in Qatar is implementing a shelter-in-place for all personnel," the statement said. "We recommend all Americans do the same until further notice."
Officials said that Americans should "find a secure location within your residence or another safe building. Have a supply of food, water, medications, and other essential items."
The Israel Defense Forces said air raid sirens were turned on across Israel proactively to alert the public to the possibility of missiles being launched toward the country.
In addition, the Israeli transport ministry confirmed Israel has closed its airspace.
As smoke was seen rising over Tehran on Saturday morning following a preemptive strike from Israel, eyewitnesses have described a series of blasts throughout the Iranian capital.
According to witnesses, there was an initial round of explosions followed by a second round of blasts at approximately around 10 a.m. local time.
