Russia has warned the US-led coalition fighting in Syria that it will view its aircraft as targets, after a Syrian military plane was shot down.
The coalition said it had shot down the Syrian Su-22 after it bombed US-backed fighters in Raqqa province on Sunday.
Russia, Syria's main ally, said it was also halting communications with the US aimed at preventing air incidents.
Syria condemned America's "flagrant attack", saying it would have "dangerous repercussions".
The incident comes at a time when the US-led coalition and the fighters its supporting on the ground are battling to oust so-called Islamic State (IS) from its Syrian stronghold of Raqqa.
"Any aircraft, including planes and drones belonging to the international coalition operating west of the Euphrates river, will be tracked by Russian anti-aircraft forces in the sky and on the ground and treated as targets," the Russian defence ministry said.
It denied the US had used a communications channel before the Su-22 fighter bomber was downed.
The memorandum of co-operation with the coalition aimed at preventing air incidents and guaranteeing flight safety was ending as of Monday, the defence ministry added.
It is not the first time communications have been suspended between the two sides - in April, the hotline was shut down after the US launched 59 Tomahawk cruise missiles at Syria's Shayrat airbase in response to a suspected chemical weapons attack on a rebel-held town in Idlib province.
But the US and Russia had agreed to resume communications last month.