Washington, Feb 8 (IANS): The US-led coalition fighting the Islamic State (IS) conducted air and artillery strikes against pro-regime forces in Syria, killing over 100 fighters, the coalition said in a statement.

The coalition said Wednesday's strikes had been carried out after forces allied with Syrian President Bashar al-Assad "initiated an unprovoked attack" against a well-established Syrian Democratic Forces (SDF) headquarters where coalition advisers were working with US-backed Syrian fighters, reports CNN.

Some 500 pro-regime troops carried out the attack using artillery, mortar fire and Russian-made tanks, a US military official said.

"Syrian pro-regime forces maneuvered T-54 and T-72 main battle tanks with supporting mortar fire in what appears to be a coordinated attack on the SDF approximately 8 kilometre east of the Euphrates River de-confliction line in Khusham, Syria," the military official said.

The official said that about "20 to 30 artillery and tank rounds landed within 500 metres" of the headquarters before the US-backed fighters and coalition forces "targeted the aggressors with a combination of air and artillery strikes".

"We estimate more than 100 Syrian pro-regime forces were killed while engaging (Syrian Democratic Forces) and Coalition forces.

"This action was taken in self-defence," the official said, adding that pro-regime forces that returned across the river had not been targeted.

No coalition or US personnel were killed or injured, CNN quoted the coalition as saying.

The official said the coalition suspected that pro-regime forces were attempting to seize territory, including lucrative oil fields, that the SDF had liberated from the IS in September.

The US maintains about 2,000 troops in Syria, who mostly work with the over 50,000-strong SDF, a mix of Kurdish and Arab fighters who have been the primary US-backed force fighting the IS in Syria.