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WASHINGTON (CBSNews.com) -- There is growing concern in the U.S. military that Russia could move to take more of Ukraine after annexing Crimea last week. No one knows what Vladimir Putin will do next, but U.S. intelligence analysts believe he may be preparing to carve out a land corridor connecting Russia directly with Crimea.

Analysts have been watching the buildup of Russian infantry, armored and airborne units for more than a week and estimate they are positioned to seize three cities in eastern Ukraine -- Kharkiv, Luhansk and Donetsk -- all of which have a predominantly Russian-speaking population.

That would give Putin a direct route by which he could keep his newly acquired province of Crimea resupplied.

In a phone conversation, Defense Secretary Chuck Hagel asked the Russian defense minister to explain the purpose of the buildup, which Russia officially calls an exercise.

"He told me that they had no intention of crossing the border into Ukraine," Hagel said.

But no one is taking that to the bank, since Putin is the one calling the shots, and it's not clear what influence, if any, the Russian defense minister will have.

But CBS News' Charlie D'Agata reports the scenario in Washington does not fit the situation he has seen on the ground in Donetsk.

CBS News has been to the border outside Donetsk and saw Ukrainian forces digging in and some Ukrainian military units moving toward the border. CBS News did not see any Russians.

The pro-Russian demonstrations that have taken place over the past two weeks have died down in recent days -- an important development, because the Russians have said that if there was a sense that ethnic Russians along the eastern region were threatened, that would give the military the green light to move in.

Therefore, if that was in any way a pretext for the Russian military to advance as it did in Crimea, that would not be justified along Ukraine's eastern border with Russia.

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