Russia will treat all U.S. aircraft west of the Euphrates river in Syria as legitimate targets after a U.S. F/A-18 shot down a Syrian regime aircraft Sunday striking anti-ISIS forces, Moscow announced Monday.

Russia also announced it would cut an established channel with the U.S. to de-conflict aerial operations over Syria. “Any aircraft, including planes and drones of the international coalition, detected in the operation areas west of the Euphrates River by the Russian air forces will be followed by Russian ground-based air defense and air defense aircraft as air targets,” Russia’s defense ministry declared.

The threat may bear little teeth given that the majority of U.S. anti-ISIS operations occur east of the Euphrates river. ISIS’s capital of Raqqa is east of the river and is currently the site of intense U.S. operations in support of its allies push on the city.

The U.S. shot down the Syrian regime aircraft Sunday after repeated warnings to stop engaging U.S. backed anti-ISIS fighters. The U.S. anti-ISIS mission noted that it repeatedly warned pro-Syrian regime forces over the now cut de-confliction channel. After this failed the Pentagon said a “Syrian regime SU-22 dropped bombs near SDF fighters south of Tabqah and, in accordance with rules of engagement and in collective self-defense of Coalition partnered forces, was immediately shot down by a U.S. F/A-18E Super Hornet.”

Russia disputed the U.S. characterization of events saying the Syrian regime aircraft was actually conducting aerial operations in support of pro-regime operations against ISIS. Russia, Iran, and Syria frequently characterize any force that is not explicitly pro-regime as terrorist to justify targeting them aggressively.

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