Russia slammed that move as a violation of international law.
"There are about a couple hundred both coalition forces and our partnered forces [near al-Tanf]. We have increased our combat power in that area," Dillon told reporters.
Dillon noted that the pro-government force that is currently well within the deconfliction zone, is a "small element," but it is not moving out of the area.
"We have not established a deadline for them [to leave]. We have made it very clear, however, that their presence inside of the deconfliction zone is considered a threat," he said.
Another cause of concern has been the buildup of pro-government forces that are outside the deconfliction zone, Dillon added.
Late May, the US Department of Defense said that pro-Syrian government militia that was attacked by a US-led coalition strike near al-Tanf last week has halted its advance and is working on leaving the deconfliction zone.
The United States and Russia signed the bilateral memorandum of understanding in October 2015 to ensure their flight safety during combat missions over Syria.