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(CNN) - Based on intelligence, Obama administration officials are very concerned the Russians are not being truthful when they say their forces near Ukraine's eastern and southern borders are merely there for training exercises, sources tell CNN.

Officials assess that Russia – as early as coming days – could use any number of pretexts to justify further military incursions into Ukraine.Moscow could express a need to protect Russian-speaking Ukrainians, or to protect transportation lines from Russia to Crimea, or the energy supply to Crimea from the rest of Ukraine.

Russian troops could accomplish this quickly, officials say, and theoretically before any other nation could even raise objections.

Officials publicly attribute their skepticism to previous Russian assurances that proved false, and privately say intelligence from the field has bolstered this view of Russia's plans.

Asked for a candid assessment of White House worries about a possible incursion, a senior administration official told CNN, "We're very concerned, but it's by no means certain."

Separately, National Security Adviser Susan Rice said Friday the Russians have said they intend to conduct military exercises.

"Obviously given their past practice, and the gap between what they have said and what they have done, we are watching it with skepticism," she said.

Defense Secretary Chuck Hagel spoke with his Russian counterpart, Defense Minister Sergey Shoigu, on Thursday.

Shoigu provided "assurance that the troops he has arrayed along the border are there to conduct exercises only, that they had no intention of crossing the border into Ukraine, and that they would take no aggressive action," according to a Pentagon read-out of the call.

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