No one was injured, according to the Qatari Ministry of Defense.
President Donald Trump announced "major combat operations" against Iran on Saturday, with daytime strikes in the joint U.S.-Israel attack targeting military and government sites, officials said.
On Sunday, Iranian state television confirmed that Ayatollah Ali Khamenei was among those killed by airstrikes in Tehran on Saturday.
Iran is responding to the U.S.-Israeli operation with missile and drone attacks targeting Israel, regional U.S. bases and Gulf nations. American diplomatic facilities have also been attacked.
In Lebanon, Israel is intensifying its long-running strike campaign against the Iranian-aligned Hezbollah militia.
Watch special coverage on Nightline, "War with Iran," each night on ABC and streaming on Disney+ and Hulu.
In a security alert released Monday, the State Department advised Americans in Iran to shelter in place, but also said, “if safe to do so, Americans should leave Iran now by land.”
"Be aware the Iranian government may restrict or prevent departures from Iran. U.S.-Iranian dual nationals must exit Iran on Iranian passports,” the alert said.
-ABC News' Shannon Kingston
U.S. Central Command said on social media that B-1 bombers "struck deep inside Iran to degrade Iranian ballistic missile capabilities” on Sunday night. This follows the use of B-2 stealth bombers on Saturday night.
Eighteen U.S. service members have been seriously wounded as part of the military operation against Iran, according to CENTCOM spokesperson Capt. Tim Hawkins.
Earlier Monday, CENTCOM announced the death of a fourth U.S. service member. Two U.S. officials have told ABC News that the service member was killed in Sunday's attack in Kuwait.
At Monday morning’s Pentagon news conference, Joint Chiefs of Staff Chairman Gen. Dan Caine told reporters that the fatalities and injuries had come from an attack on a tactical operations center.
-ABC News' Luis Martinez
President Donald Trump said at the White House that he grieves for the four U.S. service members who have been killed.
“In their memory, we continue this mission with ferocious, unyielding resolve to crush the threat this terrorist regime poses to the American people,” he said.
“We have the strongest and most powerful -- by far -- military in the world, and we will easily prevail,” he said.
As for a timeline, Trump said the U.S. is “already substantially ahead of our time projections, but whatever the time is, it's OK.”
“From the beginning, we projected four to five weeks, but we have capability to go far longer than that,” he said.
