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 U.S. embassy in Amman, Jordan, announced it is canceling appointments until further notice and has instructed personnel to shelter-in-place.
"The embassy has instructed all Mission personnel to shelter-in-place. We recommend all Americans do the same," the alert on Wednesday said.
The Al Udeid Air Base -- the largest U.S. military base in the Middle East -- was struck by a ballistic missile from Iran, the Qatari Ministry of Defense said early Wednesday local time.
No one was injured, it said.
A second ballistic missile launched from Iran was intercepted, the defense ministry said.
Secretary of State Marco Rubio said the "intensity" of the U.S.-Israeli attacks on Iran will ramp up in the coming hours and days.
"You're going to really begin to perceive a change in the scope and the intensity of these attacks as, frankly, the two most powerful air forces in the world take apart this terroristic regime and defang it and take away its ability to threaten its neighbors or hide behind the zone of immunity that allows them to develop nuclear ambitions," Rubio told reporters earlier Tuesday.
The top U.S. commander in the Middle East said the U.S. has "severely degraded" Iran's air defenses, destroying hundreds of Iran's ballistic missiles, launchers and drones.
"In simple terms: We're focused on shooting all the things that can shoot at us," Admiral Brad Cooper said in a video update on the operation released Tuesday.
Cooper said the U.S. has also destroyed 17 Iranian ships, including an Iranian submarine.
More than 50,000 troops are participating in the operation, which has so far struck nearly 2,000 targets in Iran, he said.
"Many of you may remember the Shock and Awe strikes of 2003. The first 24 hours of this operation were nearly doubled the scale," he said.
Cooper referenced the first U.S. use of one-way drones, saying, "I'd like to point out these drones were originally an Iranian design. We took them back to America, made them better and fired them right back at Iran."
Cooper said the U.S.' combat power is "building" while Iran's is "declining."
"My overall operational assessment is that we are ahead of our game plan," he said.
