Russia is preparing an "asymmetrical" response to the increase in NATO troop rotations in Eastern Europe, and in particular what it called U.S. military presence near Russian borders, Russia's ambassador to NATO said on March 31.

"Of course, our response will be completely asymmetrical. It will be calibrated to match our ideas about the degree of military threat, to be most efficient and not overly expensive," Aleksandr Grushko said on Rossia-24 television.

"We can see that the United States continues to increase its military presence in Europe with an emphasis on the eastern front... We are not passive observers. We consistently implement all those military measures that are necessary to offset this totally unjustified increased military presence," he said.

In an interview with the Izvestia newspaper, Grushko said that Russia views the troop rotations as a violation of a 1997 NATO-Russia agreement in which NATO said that "in the current and foreseeable security environment," it would refrain from "additional permanent stationing of substantial combat forces" in eastern member states.

"We've said more than once that an indefinite rotation is in no way different from permanent deployment," he said.

NATO says it has not violated the 1997 Founding Act, and that both NATO-supported infrastructure and reinforcements are "explicitly permitted" by the pact.

Western officials also say Russia has altered the security environment in Europe by seizing Crimea -- in part through military force -- and backing separatists in a deadly conflict in eastern Ukraine. Those Russian actions have prompted NATO to take additional steps to ensure security of members in the east.

Based on reporting by TASS and Interfax