Soldiers guard the Crimean parliament in Simferopol next to a sign that reads: "Crimea Russia." Getty Images Russia has roughly doubled the number of its battalions near the

Ukrainian border, Western officials said Monday, and could respond to the Kiev government's gains there by launching a cross-border incursion with little or no warning. 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

fear, may also be developing the option to intervene more directly if it appears that the pro-Russian separatists in Ukraine are on the verge of defeat. Read MoreUkraine optimism amid fierce

Donetsk fighting 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." Another

senior American official added, "The more success Ukrainian forces have, the more pressure there is on Moscow to escalate." Adding to the concern, the buildup coincides with a

newly announced Russian air force and air defense exercise. When it intervened in Crimea this year, Russia used amilitary exercise to mask its preparations. The Russian moves suggest that

the Kremlin and the West are each responding to the standoff over Ukraine by turning to the tools they know best. For President Obama and European leaders, the tool is calibrated economic

sanctions, targeted to affect banks close to the Kremlin or narrow sub-sectors of the Russian economy, like Russia's long-term ability to develop new Arctic, deep sea and shale oil

reserves. Read MoreUkraine accuses pro-Russian rebels of destroying evidence Butfor Mr. Putin, the tool is the Kremlin's ability to marshal raw military power and, increasingly, its

willingness to use it. Less than a week after the Obama administration and European nations announced new sanctions, the Kremlin has expanded its military ability to provide cover fire for

the separatists by firing artillery and rockets across the border into Ukraine. And it holds out the possibility of intervening directly. 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. Ukrainian soldiers guard a roadblock along the highway on April 24, 2014 near Slovyansk, Ukraine. Getty Images But the risks to Mr. Putin of intervening, General Clark

noted, include tougher Western economic sanctions, resistance by the Ukrainian forces and Western military support for the Kiev government. "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 tohave made the political decision to do this, perhaps

because he recognizes thatthe risks after an intervention are incalculable." If Mr. Putin did decide to intervene, one plausible outcome highlighted by General Clark and Phillip A.

Karber, a former adviser to Defense Secretary Caspar W.Weinberger, is the possibility of a "peacekeeping" intervention at the request of the Ukrainian separatists whom Moscow has

been arming and supporting politically. MORE FROM THE NEW YORK TIMES: In a Fatal Instant, a Gaza Psychologist Switches Roles Israel Moves to Wrap Up Gaza Military Operation Inquiry Finds

'Culture of Violence' at Rikers In a closed-door briefing for Congress last month, Mr. Karber noted that Russian military vehicles bearing the Russian emblem for peacekeeping

forces had been positioned close to Ukraine. Amateur video posted on YouTube and Twitter appears to show some of these vehicles operating in Ukraine. 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. Strikingly, the White House has been taking the idea of a Russian "peacekeeping" intervention seriously. "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." A spokesman for the Russian

Foreign Ministry last month insisted that the Kremlin had no plans to send peacekeeping forces to Ukraine, according to a report by the Russian news agency Itar-Tass. The Russian buildup

comes as the Ukrainian military has been gaining ground in its struggle with the separatists and amid other signs of Russian military activity. The air exercise announced by Russia's

Defense Ministry will involvemore than 100 aircraft, including attack planes, bombers and helicopters. The exercise is scheduled to last through Friday. Read MoreNew sanctions for

Russia—here's where they'll hurt On June 27, Mr. Putin also issued a decree appealing for reservists to report for up to two months of training, a step that was recently reaffirmed

by the Defense Ministry. American and NATO officials have not publicly provided precise troop estimates for the Russian forces near Ukraine. Last week, Pentagon officials put the total at

10,000 to 13,000 troops, but they acknowledged the difficulty of counting Russian troops with precision as units have moved in and out of the border region and employed camouflage to

disguise combat equipment. During a visit to Kosovo last week, Gen. Philip M. Breedlove, the top NATO commander, said the number of Russian troops along the border was growing and was

"well over 12,000." "The number of battalion task groups," General Breedlove said, "Spetsnaz units, air defense units, artillery units are all increasing." One

Western official familiar with the intelligence said Monday that it could be as high as 21,000. The officials all spoke on condition of anonymity because intelligence assessments of Russian

military strength are classified. Read MoreEU hits Russia with new military, financial sanctions Even as tensions have grown, Mr. Obama has signaled his interest in a political solution. In

a call Friday to Mr. Putin, Mr. Obama "reinforced his preference for a diplomatic solution to the crisis in Ukraine" and agreed to keep open "the channels of

communication," the White House said in a statement. Mr. Obama also repeated his "deep concerns about Russia's increased support for the separatists in Ukraine," the

White House said.