Washington: US-led coalition forces in Syria are reinforcing their garrison near the crucial Al Tanf border crossing amid a build-up of pro-government troops nearby, a US military official said on Thursday.

“We have increased our presence and our footprint and prepared for any threat that is presented by the pro-regime forces,” Colonel Ryan Dillon, spokesman for the coalition in Baghdad, told journalists by video conference.

Al Tanf, on the key highway connecting Damascus with Baghdad, has been menaced by a surge of Iran-backed troops loyal to Syrian President Bashar Al Assad. The area, also just northeast of the Jordanian border, is used by coalition forces as a training and staging area for attacks against Daesh.

Coalition planes on May 18 pounded the front of a government convoy that had apparently been headed toward the Al Tanf garrison.

That strike occurred inside an established “deconfliction zone” covering a 55 kilometre radius around the base. The Pentagon said it appeared that the Syrian government troops were trying to build a “fire base” for artillery units inside the zone.

The deconfliction zones are agreed upon between Russia, which is supporting Al Assad, and the coalition, and are designed to stop either side inadvertently striking the other’s forces on the ground and in the air.

Last weekend coalition planes dropped leaflets warning the pro-regime troops to stay away.

Dillon said there are about 200 coalition and partner forces in the area and that support for them has been beefed up.

“We have constant coverage over our forces there in Al Tanf,” he said.

The development comes in the context of growing tension over which forces will take on Daesh in Syria’s east.

Al Assad’s army is trying to prevent US-backed forces from leading that fight.