Syrian troops and allied militia in the south-east Syrian desert were seen securing their positions near the Tanf base, at Iraqi-Syrian border on Saturday (June 10), Syrian army handout video showed.

The first units of pro-Assad alliance reached the Iraqi border on Friday (June 9), near the base where U.S. special forces train local rebels to fight Islamic State, a pro-government commander said.

The advance follows an escalation of tensions between the United States and the Syrian government with its backers over control of the southeastern frontier with Iraq.

The area is seen as crucial to Assad’s Iranian allies and could open an overland supply route from Tehran to Iraq, Syria and Lebanon – a major concern to U.S. allies in the region.