The Pentagon has identified eight staging areas dotted along the border, suggesting Russian strongman Vladimir Putin is preparing for new incursions.
Among the units deployed to the area are armoured vehicles, tanks and fighter jets.
Navy Captain Danny Hernandez told the Washington Free Beacon that the exercises were being closely monitored.
He said: “We are extremely concerned about the increasing tensions near the administrative boundary between Crimea and the rest of Ukraine.
“We urge both sides to avoid provocative steps or rhetoric that could escalate the situation.”
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