Airstrikes in Syria kill 22 in Joe Biden's first military act as president.
Strikes against Iran-backed fighters were retaliation for attack on US-led forces in Iraq, says Pentagon.
The US military has carried out an air strike targeting Iran-backed militias in Syria, in the first military action undertaken by the Biden administration.
The Pentagon said the strike was ordered in response to attacks against US and coalition personnel in Iraq.
The US action destroyed "multiple facilities" used by Iraqi militant groups, it said.
Militia officials said one person had been killed but a war monitor reported at least 22 fatalities.
The Pentagon said its strike near the Iraqi border in eastern Syria was a "proportionate military response" that was taken "together with diplomatic measures", including consulting coalition partners.