Right now, there are as many as 20,000 Russian troops, complete with tanks and artillery, positioned just to the east of Ukraine. But other units deeper inside Russia appear to be on the move and could soon double the number of troops at the border.
On Thursday, Defense Secretary Chuck Hagel called Moscow to ask Russia's defense minister directly about the troop movements.
"It's an exercise only, no intention of crossing the border into Ukraine, not going to take any aggressive action," Kirby said. "That was Minister Shoigu's words, and Secretary Hagel's expectation is that he will live up to those words."
The obvious next question is: How long will the exercise last? When Hagel asked that of the Russian defense minister, the answer was that there is no firm timetable.