US concern growing over pro-Syrian regime forces amassing near coalition training facility

The Pentagon called the forces "unacceptable and threatening."

This deconfliction zone extends about 34 miles out from the city center and the Pentagon says a small pro-regime force is inside the zone while a larger pro-regime force is patrolling around the zone's perimeter. Both groups are northwest of Tanf.

"We have increased our presence and our footprint and prepared for any threat that is presented by the pro-regime forces," Dillon said.

He wouldn't indicate how many pro-regime forces make up these groups but U.S. officials have previously said that they number in the low hundreds.

According to Dillon, there are "a couple hundred" U.S., coalition and partner forces at Tanf. He said the U.S. has also increased its presence, including combat power, in that area and that it continues to fly combat air patrols.

The Pentagon considers the groups "unacceptable and threatening" to coalition forces, Dillon said, adding: "We are prepared to defend ourselves if pro-regime forces refuse to vacate the deconfliction zone."

On May 18, the U.S. dropped munitions on the group outside the zone after they entered the zone and refused to return back outside. The militia ignored Russian requests to halt its advance and ignored a "show of force" from coalition aircraft and warning shots.

Now, the U.S. has used its deconfliction channel to communicate with the Russians once again that it wants those forces out of the zone, Dillon said. Separately, a U.S. official confirmed that the Russians have passed on the message to the pro-regime forces.

Over the weekend, the U.S. dropped 90,000 leaflets on the forces inside the deconfliction zone, asking them to "leave the area immediately," according to an English translation of the leaflet provided by the Pentagon to ABC News.

Dillon would not comment on whether another U.S. strike on the group was imminent.