Nato will consider increasing its military presence near Belarus if Russian troops do not leave the country after joint exercises this month.
The alliance believes Russia has stationed 30,000 troops in Belarus, which borders Ukraine to the south.
It also borders Nato members including Poland, where a detachment of US troops arrived last week, and Lithuania, which could receive a larger German troop contingent under plans being considered in Berlin.
Further deployments from Nato allies are possible, said Rob Bauer, a Dutch admiral who heads the alliance's top military strategy committee.
"Yes, we are looking at it. There might be changes in the future as a result of these developments," Mr Bauer told a news conference in Vilnius, Lithuania.
"It very much depends, of course, on whether the Russian troops in Belarus remain in Belarus."
The movements into Belarus add to the masses of Russian troops assembled within striking distance of Ukraine, exacerbating fears of an invasion. Belarus is also accused of pestering its western neighbours Poland, Lithuania and Latvia with truckloads of migrants.
Mr Bauer said Russia's most recent deployments included field hospitals and other auxiliary units needed to support a military assault, although he said Moscow's military intentions remained unclear.
"If you're really considering an invasion, you actually need more than the fighting forces," he said.
"And that is something we also see more and more gathered along both the borders with Ukraine and Belarus. That in itself is very concerning."