The United States launched an airstrike against pro-Syrian forces who advanced inside a "de-conflicted zone" Thursday even though Russia urged them not to, the U.S.-led coalition said.

There was no immediate word on casualties in the strike northwest of the coalition base at Al Tanf in the southern part of the country.

The airstrike appears to be the first publicly known direct confrontation between U.S. forces and fighters loyal to Syrian President Bashar al-Assad. While U.S. forces have engaged with ISIS fighters — including a month ago at Al Tanf — they have not previously been reported to have directly engaged with elements of the Syrian regime or militias allied with it.

U.S. carriers fired Tomahawk cruise missiles at a Syrian military airfield in Homs on April 7, but the Defense Department said that the missiles were launched in response to an earlier Syrian chemical weapons attack and that they were timed to strike when few, if any, people were at the base.

The coalition said Thursday's airstrike was launched because the Syrian forces posed a direct threat to U.S. and allied rebel forces by advancing into the de-confliction zone near Al Tanf — despite "apparent attempts" by their Russian backers to dissuade them, as well as a "show of force" from coalition aircraft and the firing of warning shots.

The region around Al Tanf, near the border with Jordan and Iraq, is considered a de-conflicted zone under an agreement between the United States and Russia, the U.S.-led coalition said.