A drone launched by “pro-Syria regime” fighters attached U.S. backed forces in Southern Syria on Thursday, prompting a U.S. warplane to shoot it down.

This incident is the latest in a number of altercations that have placed Iranian backed militias that support Syrian President Bashar al Assad against fighters trained by the United States, the U.K. and other coalition nations.  These clashes have many concerned that a proxy war is developing in Syria, as groups with backing from several foreign nations continue to engage ISIS and increasingly, one another.

The drone, which U.S. officials claim appeared to be Iranian made, began dropping munitions near Syrian fighters who were accompanied by advisers from the American-led coalition.  The ordnance missed, and the drone was promptly shot down by a U.S. F-15E fighter.

The United States Central Command, which oversees military operations in the Middle East, said the drone destroyed on Thursday was similar in size to the U.S. MQ-1 Predator and still had unspent munitions on board when it was shot down, meaning it still posed a direct threat to Syrian fighters and U.S. advisors.

Earlier in the same day, coalition forces destroyed two pro-regime armed technical vehicles as they advanced toward coalition positions at At Tanf, inside the established deconfliction zone intended to prevent such altercations from occurring.  The vehicle’s advance “threatened coalition and partner forces,” the U.S. military claimed in a statement released on Thursday.

“The coalition’s mission is to defeat ISIS in Iraq and Syria,” the statement said. “The coalition does not seek to fight Syrian regime, Russian or pro-regime forces partnered with them. The demonstrated hostile intent and actions of pro-regime forces near coalition and partner forces in southern Syria, however, continue to concern us, and the coalition will take appropriate measures to protect our forces.”

The positions U.S. forces took action to protect in At Tanf, the statement notes, have been occupied by coalition forces for more than a year, and that its presence is intended only to combat the global threat posed by the Islamic State out of Syria.  ISIS claims the Syrian city of Raqqa as its defacto capital.  According to the Pentagon, their presence in the region to train Syrian fighters is required because of the Syrian government’s inability to root out ISIS on its own.

“As long as pro-regime forces are oriented toward coalition and partnered forces, the potential for conflict is escalated,” the statement said. “Coalition forces are oriented on ISIS in the Euphrates River Valley. The coalition calls on all parties to focus their efforts in the same direction to defeat ISIS, which is our common enemy and the greatest threat to regional and worldwide peace and security.”