Stressing that the important thing now was to prevent situations from spiralling out of control, Stoltenberg urged Russia to be transparent about military activities, to reduce tensions and prevent an escalation.
The Russian troop movements have over the past days spurred fears of a possible attack. Moscow has dismissed such suggestions as inflammatory and complained about increasing activity in the region by the NATO transatlantic alliance.
Russia annexed Crimea from Ukraine in 2014 and says the waters around it belong to Moscow now, despite most countries continuing to recognize the peninsula as Ukrainian.
Russian-backed separatists took control of Ukraine's eastern Donbass region that same year and soldiers on both sides continue to be regularly killed in the conflict there.
The troop border build-up - which Ukraine last week estimated at 100,000 - was dangerous, said Stoltenberg, because it reduced the amount of warning time, should Russia decide to "conduct a military aggressive action against Ukraine."