A coalition airstrike in Syria on Thursday targeted pro-regime forces who were threatening a coalition base where advisers train anti-Islamic State fighters, the U.S.-led coalition command said.
The forces came within a 34-mile defensive zone around the at-Tanf base in southern Syria, according to the command.
U.S. military officials have not yet determined if Syrian army forces were targeted in the strike or if they were militias aligned with the government of Syrian President Bashar Assad.
The airstrike targeted a tank and two earth-movers that were building fighting positions within the defensive zone, according to a military official who was not authorized to speak publicly about the strike. The Pentagon did not provide an estimate of casualties.
Officials from the U.S.-led coalition attempted to use a hotline established with the Russians to warn the Syrian government to remove the forces from near the coalition base. The Russians relayed the message to the Syrian government, but the forces did not withdraw, the official said.
The coalition aircraft also used warning shots in an effort to dissuade the forces from establishing the fighting positions near the base.
The U.S.-led coalition is battling the Islamic State, also known as ISIS, in Syria and is generally not attacking Assad. An exception was the U.S. cruise missile strike that was launched last month in retaliation for Assad’s chemical weapons attack that killed 86 people.
Several hundred U.S. advisers are in Syria to help train and organize a loose network of opposition groups, called the Syrian Democratic Forces, to defeat ISIS. The U.S.-led coalition is also supporting the force with airstrikes.
Aside from the April cruise missile strike, the coalition-backed forces have avoided contact with Assad regime forces.
The coalition has described Thursday's strike as a defensive action that does not change the U.S. policy of focusing military efforts on defeating ISIS.
But it highlights the growing complexity of the battlefield in Syria, where multiple forces are engaged in fighting for varying reasons and amid ever-shifting alliances. The complexity heightens the risk an incident that could draw the United States and Iran deeper into the conflict.
As the Islamic State is pushed out of Iraq and Syria by U.S.-backed forces Iran has grown increasingly concerned about American influence in the region.
Iran has started using the militias they support, including Hezbollah, a Lebanese-based group, to attack the SDF, said Ahmad Majidyar, an analyst at the Middle East Institute.
"They are testing U.S. military resolve," he said.
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