The Iranian foreign ministry late Saturday condemned "the belligerent and terrorist attack of the American army on civilian targets" in the eastern Syrian region of Deir Ezzor.

Washington said it launched the retaliatory raids after a US contractor was killed -- and another contractor and five military personnel wounded -- by a drone "of Iranian origin" that struck a US-led coalition base in Syria on Thursday.

The Syrian Observatory for Human Rights war monitor has said at least 19 people, most of them Syrian, were killed in the following US strikes launched early Friday morning.

The US strikes triggered further rocket attacks by Iran-backed militias, the Observatory said.

The Syrian government, which is closely allied with Iran, has accused the United States of lying about the targets of its air strikes to justify its "act of aggression."

The United States has about 900 troops in posts across northeastern Syria to keep pressure on the remnants of the Islamic State group and support the Kurdish-led Syrian Democratic Forces, which control most of the northeast.

US personnel in Syria have frequently been targeted in attacks by militia groups that Washington says are backed by Tehran.

Iranian foreign ministry spokesman Nasser Kanani said the US troops' presence was "illegal" and a "violation of international law and the national sovereignty" of Syria.

In contrast, Kanani said Iranian forces were in Syria at the behest of Damascus.

"The military advisers of the Islamic Republic of Iran are present in Syria at the request of the Syrian government", he said, adding that they will continue to support Damascus to "help establish peace, stability and lasting security."