It's not a 'threat', it's an explanation
Russians are saying that since February 25 they have been trying to get US' practical cooperation on sustaining the cesefire regime (proclaimed Feb 22) in Syria, but that the Americans have not moved a finger in that direction.
Moreover the Russians are adding that if US does not show interest in helping out, they will have no choice but to resort to dropping bombs on violators:
Russia is ready to unilaterally use military force starting March 22 against the armed groups, systematically violating Syria’s ceasefire, if it gets no US response to its proposals on controlling the ceasefire regime, head of the Main Operations Directorate of the Russian General Staff Sergei Rudskoi said on Monday.
He said that Russia had sent the corresponding proposals on February 25.
"If the Russian Federation gets no response from the United States to these proposals, it will proceed starting March 22 to unilateral application of the rules specified in the Agreement," Rudskoi said.
American media has interpreted this as a Russian "threat" but really it is a Russian explanation of what they'll do and why.
Russians are saying they are seeing "systematic" violations of the ceasfire by rebel groups allied to the Nusra Front that were armed and otherwise backed by the United States.
The Russians therefore presume that the US wields some influence with them and that they would be more willing to adhere to the ceasefire closely if US along with Russia took practical steps to persuade them to do so.
However the Russians themselves wield no special influence with these groups and so the only way they can possibly compel them to adhere to the ceasefire they signed up for is by using military force. (Ie, the same way they forced them to sign up for the cessation of hostilities in the first place.)
The only real positive is that Americans have denied dragging their feet on coming aboard:
“Whoever is making such statements must be misinformed, because these issues have been discussed at length already, and continue to be discussed, in a constructive manner,” Col. Steve Warren, a spokesman for the U.S.-led coalition in Iraq and Syria, said in an email.
“We fully expect Russia to refrain from unilateral actions and respond to our counter-proposal” about how to put the truce agreement into effect, he said.
If Americans are indeed sincerely interested in helping keep the ceasefire alive, as they say they are, that would be the best news yet. However, their protestations do not seem to match their actions:
The United States on Monday rejected Russia's call for an urgent meeting over violations of Syria's three-week cessation of hostilities, saying that its concerns were already being handled in a constructive manner.
Why would Americans if they were truly interested in helping maintain the ceasefire spurn a call for an urgent meeting of the obviously concerned and nervous Russians?