U.S. officials warn of possible Russian military incursion into Ukraine

U.S. intelligence officials are warning European allies of a potential Russian military incursion into Ukraine, with the likelihood of one increasing as the weather gets colder. An incursion is weather-dependent, but could happen in a matter of weeks, barring intervention from the West, U.S. officials briefed on the matter told CBS News.

Tens of thousands of Russian troops have amassed at the Ukraine border -- months after thousands of troops assembled at the border in the spring. But unlike last spring's buildup, which was regarded as a show of force, U.S. intelligence officials are warning that this one could be in preparation for an actual incursion into the Donbass region of Eastern Ukraine.

NATO Secretary General Jens Stoltenberg said on Friday the alliance was closely monitoring a "large and unusual concentration of Russian forces" near Ukraine's border.