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A Russian military transport truck and some 150 soldiers crossed into Crimea from southern Russia at the key port of Kerch, Ukrainian television reported on Tuesday.
Ukrainian officials also said Russia was building up armor on its side of the 2.7- mile wide Kerch strait between Crimea and southern Russia.
Russian forces shipped three truckloads of troops by ferry into Crimea after taking control of the border post on the Ukrainian side, a spokesman for Ukraine's border guards said.
Kiev's U.N. ambassador, Yuriy Sergeyev, said Russia had deployed roughly 16,000 troops to Crimea since last week.
Moscow's Black Sea fleet has a base in Crimea, which is Ukraine's only region featuring a majority of Russian speakers.
Ed Flanagan of NBC News, The Associated Press and Reuters contributed to this report.