The Pentagon said the U.S.-led coalition fighting the Islamic State terror group struck pro-government forces Tuesday in southern Syria after they entered an area near a coalition base.

The attack took place near Tanf, Syria after forces supporting Syrian President Bashar Assad entered the area with a tank, artillery, anti-aircraft weapons, other vehicles and more than 60 soldiers.

A senior U.S. defense official told Fox News the pro-regime forces got between "25 to 55 kilometers" from the well-known training base.

The U.S. then carried out multiple airstrikes on these forces after multiple warning shots were fired, flyers were dropped and two calls were placed to the Russians on a de-confliction hotline, the official said.

The U.S. military took "a variety of steps to deescalate and warn" the forces, the official said, adding they "were asked to leave."

When the forces did not withdraw, U.S. warplanes bombed them around 11 a.m. ET. The Pentagon statement didn't further identify the targeted forces.

President Trump was briefed on the incident, officials told Fox News.

In recent weeks, officials have complained about various militants refusing to leave an area near Tanf, Syria where the U.S. and its partners train Syrian rebels there to fight ISIS.

Last month, the coalition struck similarly described forces in the same area after pro-Syrian regime forces advanced "a significant distance" into the de-confliction zone.

Fox News' Jennifer Griffin, John Roberts, and the Associated Press contributed to this report.