The New York Times correspondents Michael R. Gordon and Eric Schmitt write about it, Censor.NET reports, citing the New York Times article.
Over the past several weeks, Russia has built up 17 battalions - totaling 19,000 to 21,000 troops, according to one Western estimate - into a battle-ready force of infantry, armor, artillery and air defense within a few miles of the border. In addition, it has vastly expanded its firepower, increasing the number of advanced surface-to-air missile units to 14 from eight, and deploying more than 30 artillery batteries, according to the officials.
American intelligence experts say that the advance by Ukrainian government forces on Donetsk and other steps that the Ukrainian government is taking to regain territory in the east from the separatists might prompt Mr. Putin to send his forces across the border under the guise of a "peacekeeping operation."
"That's a very real option," a senior Defense Department official said on Monday. "And should Putin decide, he could do that with little or no notice. We just don't know what he's thinking."
Wesley K. Clark, the retired general and former NATO commander, said that Mr. Putin had put the pieces in place for a major military intervention by massing Russia forces near the border, arming separatist groups, infiltrating operatives, conducting exercises to practice the military's ability to coordinate fire and supporting the self-proclaimed mayor of Luhansk, who has called for the Russian military to come to the separatists' aid.
"He has set the military and political conditions for what he believes could be a successful intervention," General Clark said. "But he still doesn't seem to have made the political decision to do this, perhaps because he recognizes that the risks after an intervention are incalculable."
Several American officials confirmed that Russian armored vehicles and trucks with the peacekeeping insignia had been seen on Russian territory near Ukraine. But these officials said that Western intelligence had no independent confirmation that they had crossed into Ukraine.
"We've seen a significant rebuild up of Russian forces along the border, potentially positioning Russia for a so-called humanitarian or peacekeeping intervention in Ukraine," Antony J. Blinken, the deputy national security adviser to Mr. Obama, said last week when the White House announced new sanctions. "So there's urgency to arresting this."