“That was the first time that we saw them actually firing or shooting at coalition forces,” Col. Ryan Dillon, the spokesman for the American-led task force that is fighting the Islamic State, also known as ISIS or ISIL.

The United States Central Command, which oversees military operations in the Middle East, said in a statement that it wanted to keep the focus on fighting the Islamic State and was not looking for a confrontation with the Assad government — or with the Russian and Iranian-backed forces that are supporting it.

“The coalition’s mission is to defeat ISIS in Iraq and Syria,” the Central Command 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.”

But some analysts say the operations by the American-backed Syrian fighters also serve the secondary purpose of denying important terrain to Iran’s proxy forces.

“Iran’s strategic goal is to capture a major border crossing and establish a land route from Syria to Iraq and, ultimately, to Iran as the Islamic State crumbles,” said Amir Toumaj, a research analyst with the Foundation for Defense of Democracies, a Washington-based policy center.

“Even though the United States is careful to say that it is only fighting ISIS, its strategy also appears to be intended to deny the Iranians that land corridor,” he said.

There is no question that the situation in southeastern Syria has heated up in recent weeks. To protect the garrison at al-Tanf, the United States has warned Iranian-backed militias and Syrian forces to stay out of the “deconfliction” zone it has declared around the base.