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The Cossacks were once the patrolmen of Russia's borderlands and now back pro-Moscow forces controlling Crimea.
The Kremlin has stuck to its line that no active-duty Russian troops were part of the apparent takeover of Crimea over the weekend. But American officials have brushed off Russia's denials almost incredulously.
Earlier, Russian Prime Minister Dmitry Medvedev said Moscow was simplifying procedures for native Russian speakers who have lived in Russia or the former Soviet Union to gain Russian citizenship.
Ed Flanagan of NBC News contributed to this report.