Russia has said it will treat US warplanes operating in parts of Syria where its air forces are also present as "targets" amid a diplomatic row caused by the downing of a Syrian jet.
The country's defence ministry said it would track US-led coalition aircraft with missile systems and military aircraft, but stopped short of saying it would shoot them down.
A hotline set up between Russia and the US to prevent mid-air collisions will also be suspended.
"All kinds of airborne vehicles, including aircraft and UAVs of the international coalition detected to the west of the Euphrates River will be tracked by the Russian SAM systems as air targets," the Russian Defence Ministry said in a statement.
The warning followed after a US F-18 Super Hornet shot down a Syrian army SU-22 jet on Sunday in the countryside southwest of Raqqa - the first such downing of a Syrian jet by the US since the start of the country's civil war in 2011.
Washington said the jet had dropped bombs near US-backed forces but Damascus said the plane was downed while flying a mission against Isis militants.
Russia's Defence Ministry said the suspension of its communication line with the Americans would begin immediately.
The US did not use its hotline with Russia ahead of the downing of the Syrian government warplane, said the ministry, which accused the US of a "deliberate failure to make good on its commitments" under the de-confliction deal.
"The shooting down of a Syrian Air Force jet in Syria’s airspace is a cynical violation of Syria’s sovereignty," the ministry said.
"The US’ repeated combat operations under the guise of ‘combating terrorism’ against the legitimate armed forces of a UN member-country are a flagrant violation of international law and an actual military aggression against the Syrian Arab Republic."
Theresa May appealed to Russia to continue the use of "deconfliction" measures over the skies of Syria to reduce the risk of misunderstandings in what is a crowded airspace.
Russia, which has been providing air cover for Syria's President, Bashar al-Assad, since 2015, has an agreement with the US aimed at preventing incidents involving either country's warplanes engaged in operations in Syria.
Downing the jet was akin to "helping the terrorists that the US is fighting against”, Sergei Ryabkov, Russia's Deputy Foreign Minister, said.