"In last week alone, several units have moved into the border region," he said.
Moscow's motivation isn't entirely clear, the military officer added, but "they are definitely capable of moving into Ukraine and dramatically influencing the situation in the east."
After Russia annexed the Ukrainian region of Crimea earlier this year, Moscow stationed about 40,000 troops along the border with Ukraine.
The move was condemned by Ukrainian and Western leaders, who said the troops were well-equipped and capable of mounting a rapid incursion into Ukraine.
Following Ukraine's presidential election last May won by Petro Poroshenko, as well as sanctions imposed by the U.S. and European Union, Russia withdrew almost all of those troops, and Russian President Vladimir Putin began making conciliatory statements toward Ukraine.
The renewed military buildup of recent weeks is causing unease among Western leaders.
"We see again a presence on the Ukrainian border by large Russian formations," Gen. Philip Breedlove, NATO's top military commander, said last week.
"Those formations are growing every day and their capability grows every day, and that is concerning."
NATO Secretary-General Anders Fogh Rasmussen, in a speech in San Francisco, said Moscow is sending contradictory signals about its intent.
"You see a kind of a double game from the Russian side," Mr. Rasmussen said.
Putin is making conciliatory statements, he added, "but then, on the other hand, we see a continuation of covert military operations and other operations in Ukraine with the aim to continue to destabilize the situation in eastern Ukraine."
NATO officials are concerned about what they see as a sort of ambiguous or shadow warfare by Russia, including propaganda campaigns and the use of separatist proxies within several countries.
Groups within Russia have been posting photos on social media that purport to shows major humanitarian problems in eastern Ukraine, the NATO military officer said, but in fact the photos come from other places and times.
"Their statements to their own citizens appear to be building a justification for interference, and that is a bit worrying to us," the officer said.
Source: The Wall Street Journal