US-led coalition launches defensive strike in Syria

"Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces headquarters" on February 7, CENTCOM said in a statement.

"Coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack eight kilometres east of the agreed-upon Euphrates River de-confliction line," the statement said.

Using an alternative acronym for IS, the coalition said it then "conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition's defeat-Daesh mission."

The statement did not say whether the incident caused any casualties.

The coalition is working to uproot the last IS fighters from the east bank of the Euphrates with the support of the SDF, an alliance of Kurdish fighters and Arabs. 

According to a verbal agreement with Moscow, Russian fighters who support President Bashar al-Assad's regime are expected to stay on the west bank of the Euphrates.

The CENTCOM statement did not specify who the aggressors were, as the term "pro-regime" could refer to Syrian pro-government forces or the Russian army - with which incidents have escalated in recent months as the combat zone narrows.

"The Coalition remains committed to focusing on the defeat-Daesh mission in the Middle Euphrates River Valley and asserts its non-negotiable right to act in self-defence," the statement said.

The Syrian regime has been carrying out a relentless bombing campaign, including the use of chemical weapons, against civilians at it aims to annihilate the last remaining rebel strongholds.

The US clashed with Syria's ally Russia on Tuesday when it strongly condemned its use of chlorine gas against Syrian civilians, including children.