Russian President Vladimir Putin has said that Ukraine's military development with Western support represents a genuine threat to Moscow.
"Formal membership [of Ukraine] in NATO may not take place, but military development of the territory is already under way," Putin said during a meeting of the Valdai Discussion Club in the Black Sea resort of Sochi.
Relations between NATO and Russia have been tense recently and official contacts have been limited since Moscow occupied Ukraine's Crimean Peninsula in 2014.
The 30-member Western alliance is also concerned over Russia's nuclear missile development, aerial intrusions into NATO airspace, and the buzzing of allied ships by Russian fighter planes.
"This really poses a threat to Russia. We are aware of that," Putin said, adding that U.S. Secretary of Defense Lloyd Austin's visit to Kyiv this week had effectively paved the way for Ukraine to join NATO.
Ukraine has expressed a desire to join NATO to protect itself against further Russian aggression. Moscow has warned that Ukraine's membership in NATO would represent a "red line."
Austin reiterated Washington's "unwavering" support for Ukraine's territorial integrity during his visit to Kyiv on October 19, and said Russia had no right to veto Kyiv's aspirations to join NATO.
Austin, who was on a three-country tour of the Black Sea region aimed at underlining Washington's support for allies and partner states, added that Russia continued to be an "obstacle" to the peaceful resolution of the deadly conflict in eastern Ukraine.
Austin's visit to Kyiv came amid continued fighting between Ukrainian government forces and Kremlin-backed separatists in a conflict that has claimed more than 13,200 lives since April 2014.
Austin also visited Georgia and fellow NATO-member Romania.
On October 21, NATO defense ministers agreed a new master plan to defend against any potential Russian attack on multiple fronts.