The latest exchange of fire underscored soaring military tensions amid diplomatic efforts between Tehran and the West to try to save Iran's 2015 nuclear deal with major powers.
It came a day after President Joe Biden authorised US air strikes in Syria on an ammunition depot and other facilities used by groups affiliated with Iran's elite Revolutionary Guards Corps (IRGC).
The US military said the suspected Iran-backed militants launched two separate attacks starting at approximately 7.20pm local time on Wednesday in Syria (3.20am Thursday AEST).
Several rockets landed inside the perimeter of Mission Support Site Conoco in northeast Syria and were quickly followed by rocket fire, it said in a statement.
That triggered a US response from helicopters that, according to initial assessments, killed two or three militants conducting one of the attacks.
Central Command said the wounded US service member had been at Mission Support Site Conoco and returned to duty after treatment.
Two other service members were under evaluation for minor injuries, it said.
"We are closely monitoring the situation," General Michael 'Erik' Kurilla, who leads Central Command, said in a statement.
The latest attacks by the suspected Iran-backed groups could be retaliation for Biden's air strikes on Tuesday.
Earlier on Wednesday, Iran denied having any link to sites targeted on Tuesday by the United States.
The strikes involved eight US fighter jets - four F-16s and four F-15Es - and hit nine targets in Syria, the military said.
US forces first deployed into Syria during the Obama administration's campaign against Islamic State, partnering with a Kurdish-led group called the Syrian Democratic Forces.
There are about 900 US troops in Syria, most of them in the country's east.
Iran-backed militias established a foothold in the western Asian nation while fighting in support of President Bashar al-Assad during Syria's civil war.
15°C