"The president gave the direction for these strikes."
Army Colonel Joe Buccino
The military's Central Command said in a statement that such strikes were aimed at protecting US forces from attack by Iran-backed groups.
It cited one such incident on August 15, which Reuters has reported involved drone attack on a compound run by coalition and US-backed Syrian opposition fighters, with no casualties.
"The president gave the direction for these strikes," said spokesman Army Colonel Joe Buccino.
Central Command called the strikes a "proportionate, deliberate action intended to limit the risk of escalation and minimize the risk of casualties."
The statement about Tuesday's US strike did not mention whether there were any casualties and did not say whether the air strikes were carried out by manned or unmanned aircraft.
History of US operations in Iraq and Syria
This is not the first time US warplanes have struck Iran-backed forces in Iraq and Syria. The United States hit operational and weapons storage facilities at two locations in Syria and one in Iraq in June last year.
US forces first deployed into Syria during the Obama's 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 east.
But Iran-backed militias established a foothold in Syria while fighting in support of President Bashar al-Assad during Syria's civil war.
Iranian-backed militias are heavily concentrated west of the Euphrates in Deir al-Zor province, where they get supplies from Iraq through the al-Bukamal border crossing.