A Russia Defense Ministry spokesman suggested that Russian antiaircraft systems may shoot down U.S. or U.S.-led coalition aircraft if they attack Syrian forces.
The warning by Major General Igor Konashenkov was the harshest and bluntest remark to date by a Russian official about the ongoing air campaigns in Syria.
It comes as bilateral ties between Washington and Moscow continue to spiral downwards. The United States earlier this week announced it was suspending talks with Russia about cooperation on Syria that were aimed at reestablishing a cease-fire that broke down on September 19.
Since then, Syrian and Russia warplanes have conducted a campaign of air strikes on rebel-held neighborhoods of Aleppo, killing hundreds of civilians.
Since the weekend, Russia has deployed S-300 antiaircraft missile systems in Syria, and has sent three warships to the eastern Mediterranean. Several longer range S-400 antiaircraft systems are also in Syria and mounted on Russian warships off Syria's coast.
The Washington Post reported on October 4 that the Pentagon this week presented U.S. administration officials with several options for possible air strikes against Syrian government forces in response to the offensive on Aleppo.
Speaking at a briefing in Moscow on October 6, Konashenkov said Moscow was worried by The Washington Post report and other similar media reports.
"I would recommend that our colleagues in Washington carefully weigh possible consequences of the fulfillment of such plans," Konashenkov said.
Konashenkov also warned that the Russian military would probably not have time to use the hotline set up earlier this year to avoid confrontation between U.S. and Russian military forces if missiles are fired at targets in Syria.
"One should realize that the Russian crews manning the air defense systems will unlikely have time to find out an exact flight path of missiles and their origin through the direct [hotline]," he said.
In Washington, U.S. State Department spokesman John Kirby criticized the Russian Defense Ministry spokesman's comments, saying they "weren’t helpful" for moving toward "some sort of diplomatic solution."
But Kirby said "the Russians should speak for themselves."
Kirby also said U.S. administration officials were discussing a range of options for action in Syria.
"Not all of those options revolve around diplomacy," he said.
With reporting by Reuters and AP