BEIRUT -- A U.S. airstrike struck pro-Syrian government forces that the coalition said posed a threat to American troops and allied rebels operating near the border with Jordan on Thursday, the first such close confrontation between U.S. forces and fighters backing President Bashar Assad.
The coalition said "apparent" Russian attempts to stop pro-Assad forces from moving toward Tanf, as well as warning shots and a show of force, had failed.
American officials and Syrian activists said the strike hit in the desert near the border with Jordan, though it was unclear if it struck the Syrian army or just militias allied with the government.
The region around Tanf, where the borders of Jordan, Syria and Iraq meet, has been considered a de-conflicted zone, under an agreement between the U.S. and Russia.
Speaking to reporters, Defense Secretary Jim Mattis said the U.S. will defend its troops in case of "aggressive" steps against them. He was asked if the airstrike increases the U.S. role in the Syrian war.
"We are not increasing our role in the Syrian civil war, but we will defend our troops," Mattis said. "And that is a coalition element made up of more than just U.S. troops, and so we will defend ourselves (if) people take aggressive steps against us."
The "defensive" strike was also an apparent signal to Assad to keep his forces out of a zone where U.S.-backed rebels are fighting the Islamic State group.
"This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south ... were unsuccessful, a coalition aircraft show of force, and the firing of warning shots," the U.S.-led coalitions aid. It said coalition forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against IS.
The U.S. strike marks a new approach in what has become an intensely crowded and complicated war zone. Thursday's strike was the coalition's first on pro-Assad forces in the battlefield. The coalition had so far kept its military operations focused on Islamic State militants and al-Qaida-linked groups.
Last month, the U.S. fired 59 missiles at a government air base in central Syria as punishment for a chemical attack blamed on Assad's forces that killed nearly 90 people.
An increasingly visible U.S. role in Syria has also raised the possibilities of friction with the various forces on the ground. The U.S. is backing Syrian Kurdish forces who are also fighting IS to the country's east.
US troops have sent patrols in the area to act as a buffer between Turkish troops and the Kurdish fighters. Turkey views the U.S-backed Kurdish fighters as an extension of its own insurgent group.
In recent days, near the border with Jordan, another set of U.S-backed rebel fighters have been on a collision course with government troops in the area of Tanf.
The government launched a new offensive in recent days in the area, and activists say pro-government militiamen, mainly from Iran and the Lebanese Shiite militant Hezbollah group, have deployed there aiming to secure the main highway that runs from Damascus to Baghdad and beyond, to Tehran.
Tensions have been building as part of a race for control of territory stretching from the provincial capital of Deir el-Zour in northeastern Syria to the Iraq border. The area gained attention as the battle for the Iraqi city of Mosul escalated in recent weeks. An estimated 10,000 IS fighters uprooted from Mosul are believed to be massing in the border area.
The U.S. officials said the American airstrike hit the pro-Syrian government forces as they were setting up fighting positions in a protected area near Tanf. They said a tank and a bulldozer were also hit.
One official said the pro-regime forces had entered a so-called "de-confliction" zone without authorization and were perceived as a threat to U.S.-allied troops there. The officials weren't authorized to speak publicly on the matter and demanded anonymity.