Syrian opposition forces backed by a US-led coalition killed at least 20 militant Islamic State (IS) fighters and took several others prisoner on Thursday, coalition military officials said.

According to the coalition fighting IS in Syria and Iraq, a vetted Syrian opposition group known as the Maghawir al-Thawra detected the fighters early Thursday in an area controlled by pro-Syrian regime forces in southern Syria, in the Al-Tanf border region.

Supported by coalition forces, the Maghawir al-Thawra “quickly and professionally conducted an operation,” the coalition said in a statement.

“This is the second time in less than a month that convoys of Daesh (IS) terrorists, fleeing the middle Euphrates River valley, transited through the regime-controlled territory,” the statement read, using an Arabic name for the IS.

“Despite the presence of Russian-backed, pro-Syrian regime forces in the area, Daesh (IS) still finds ways to move freely through regime lines and pose a threat,” coalition director of operations Brigadier General Jonathan Braga added.

The coalition did not give additional details on the captured fighters, saying only “a number of Daesh terrorists were detained in the engagement, including foreign terrorists.”

The coalition says 98 per cent of the territory once held by the IS has now been recaptured. The Maghawir al-Thawra have been fighting the IS in the tri-border area of Iraq, Syria, and Jordan since 2015.