The United States carried out airstrikes in Iraq and Syria after a deadly drone attack in Jordan killed three American troops and injured around 40 more.
The US military said the retaliatory strikes targeted 85 sites linked to Iran's Revolutionary Guards and the militias they support.
While the U.S. strikes did not target any locations inside Iran, they are likely to increase concern about tensions escalating in the Middle East from Israel's more than three-month-old war with Palestinian Hamas militants in Gaza.
The U.S. military said the strikes hit targets including command and control centers, rockets, missiles and drone storage facilities, as well as logistics and munition supply chain facilities.
"Our response began today. It will continue at times and places of our choosing," US President Joe Biden said in a statement.
"The United States does not seek conflict in the Middle East or anywhere else in the world. But let all those who might seek to do us harm know this: If you harm an American, we will respond."Syrian state media said "American aggression" on sites in Syria's desert areas and the Syrian and Iraqi border resulted in a number of casualties and injuries.
It came just hours after Biden and Pentagon leaders attended the remains of the three American soldiers killed in the Jordan attack returning to Dover Air Force Base in Delaware.
The Jordan attack was the first deadly strike against U.S. troops since the Israel-Hamas war erupted in October.
The United States has assessed the drone that killed three of its soldiers and also wounded more than 40 other people was made by Iran, U.S. officials told Reuters.
But the Pentagon had said it did not want war with Iran and did not believe Tehran wanted war either, even as Republican pressure increased on Biden to deal a blow directly against Iran.
Before the retaliatory strikes on Friday, Iranian President Ebrahim Raisi said that Iran will not start a war but would "respond strongly" to anyone who tried to bully it.
The Revolutionary Guards have scaled back deployment of their senior officers in Syria due to a spate of deadly Israeli strikes and will rely more on allied Shi'ite militia to preserve their sway there.