Russian army tanks stand ready to move back to their permanent base after drills in Russia. (Russian Defense Ministry Press Service via AP)
"It remains to be seen whether there is a Russian withdrawal ... What we see is that they have increased the number of troops, and more troops are on the way," Stoltenberg added.
On Wednesday, the Russian Defense Ministry released a video showing a trainload of armored vehicles moving across a bridge away from Crimea, the Black Sea peninsula that Russia annexed from Ukraine in 2014. A day earlier, the ministry reported the start of a pullback of troops following military exercises near Ukraine.