“The demonstrated hostile intent and actions of pro-regime forces toward coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated,” a statement from the Pentagon said, following today’s drone incident.

The fast-multiplying confrontations in Syria involving US forces could spiral into a deeper conflict with both Russia and Iran, as both countries attempt to consolidate the territories under their control as ISIS weakens. In the past week, Russia confirmed that it would treat US-led coalition aircraft flying west of the Euphrates River in Syria as “potential targets,” while Iran fired missiles from inside its territory against ISIS targets in eastern Syria.

“Iran and its allies, apparently with Russian support, are attempting to fill the vacuum left by the collapsing ‘caliphate’ of ISIS in eastern Syria,” Spyer said.

The eastern part of Iran’s land link — dubbed by some as the “Shiite corridor” — is reported to have already been secured. In a dispatch from the region earlier this month, Jerusalem-based journalist Seth Frantzman said Iraqi militias allied with Iran reached the border with Syria on May 29.

“Photos posted online showed Shia militiamen celebrating next to signs pointing towards Homs in Syria,” Frantzman wrote. “Qassem Soleimani, the leader of Iran’s IRGC Quds Force, was seen in photos near the Syrian border.

Spyer warned, “The advance of Iran’s corridor would have a major impact on the type of war Iran and its proxies could in the future wage on Israel. The weeks ahead will show whether the US and its allies on the ground will act to prevent this.”