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Russia has amassed enough forces on Ukraine’s border to reach Moldova’s vulnerable Trans-Dniester region, NATO’s top military commander warned on Sunday.

Moscow has mobilized a “very, very sizeable and very, very ready” military contingent on Ukraine’s eastern boundary, Supreme Allied Commander Europe General Philip Breedlove told a conference in Brussels.

He said he was worried the troop build-up could pose a threat to the mainly Russian-speaking separatist region, whose local pro-Russia leader has already appealed to Moscow for an annexation similar to the one carried out in Crimea.

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The president of ex-Soviet Moldova warned Russia last Tuesday against considering any move to annex Trans-Dniester.

Russia has large # of ready, capable forces on Ukraine's border @PMBreedlove Me:Ready to pounce at worst, intimidate at best #BrusselsForum - Damon M. Wilson (@DamonMacWilson) March 23, 2014

Breedlove said NATO was very concerned that the region was, in Russia's view, was the "next place where Russian-speaking people may need to be incorporated."

"There is absolutely sufficient (Russian) force postured on the eastern border of Ukraine to run to Trans-Dniester if the decision was made to do that and that is very worrisome," he said, according to Reuters.

Russian forces are once again amassed in big numbers in the neighborhood. About two divisions – some 20,000 infantry, airmen and special operations forces – are poised along Ukraine’s eastern border with Russia, ready to intervene if ethnic conflict endangers large concentrations of Russian speakers in Ukraine’s southern and eastern regions.

Russia’s Deputy Defense Minister Anatoly Antonov said Moscow was “in compliance with all international agreements limiting the number of troops in the border areas with Ukraine,” according to the official Itar-Tass news agency.

On Tuesday, Senators John McCain (R-AZ) and Lindsey Graham (R-SC) issued a statement saying the developments over Trans-Dniester were "disturbing, though not at all surprising."

"Our friends in Moldova - as well as the Baltic countries and every other country with ethnic Russian populations - are right to be concerned," their statement said. "The West must impose real costs on Russia for its aggression in Ukraine. By failing to do so, we only invite further aggression elsewhere."

NBC News' Jim Maceda, Albina Kovalyova and Reuters contributed to this report.