In response to the US downing of a Syrian fighter jet, Russia has cut a vital line of communication with US forces in Syria and is threatening to shoot down any US aircraft operating in Russian/Syrian operational airspace.

Moscow outlined the changes in Syria war policy via a terse statement by officials from the Russian Ministry of Defense released Monday, local news reports state. The moves were taken in response to Sunday’s shoot-down of a Syrian military aircraft by a U.S. fighter, according to The Washington Times.

The shoot-down took place near the Syrian town of Ja’Din, located south of Tabqa dam, the coalition’s main logistics and air support hub for the ongoing assault to drive the terror group known as ISIS or ISIL from their self-styled capital of Raqqa, says a Pentagon statement.

“Pro-Syrian regime forces” had launched an attack on Ja’Din Sunday afternoon, retaking the town from the Syrian Democratic Forces or SDF, the network of Arab and Kurdish militias tasked by the U.S. coalition to liberate Raqqa. U.S. warplanes carried out several low passes over the Ja’Din, halting the attack by forces loyal to Syrian President Bashar Assad.

It remains unclear whether the regime forces that led the assault on Ja’Din were government troops or Iranian paramilitaries sent in by Tehran to support the regime.

But after the assault on Ja’Din had ceased, a Syrian-flagged Su-22 fighter launched airstrikes on SDF fighters taking back their positions inside the city, the Pentagon said. A F/A-18 Super Hornet was sent to take out the Syrian warplane, which it did, “in accordance with rules of engagement and in collective self-defense of Coalition partnered forces,” according to the department’s statement.

It was the first air-to-air engagement carried out by U.S. forces since 1999.

“The coalition presence in Syria addresses the imminent threat ISIS in Syria poses globally,” said a Pentagon statement issued on Monday. “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,” it added.

Forces loyal to Mr. Assad and Russian forces backing the regime have taken a larger role in the fight against ISIS. Iranian paramilitaries tied to the regime have also become increasingly aggressive toward U.S. and coalition forces along the Syrian-Iraqi border in recent months.