The Kremlin said on Thursday that Ukrainian membership of NATO would be a "red line" for Moscow and that it was worried by talk that Kyiv may one day be granted a membership action plan.
Kremlin spokesman Dmitry Peskov made the remarks a day after U.S. President Joe Biden and Russian President Vladimir Putin held talks in Geneva.
Ukrainian President Volodymyr Zelensky said on Monday that he wanted a clear "yes" or "no" from Biden on giving Ukraine a plan to join the NATO. Biden said Ukraine needed to root out corruption and to meet other criteria before it could join.
Peskov said Moscow was following the situation closely. "This is something we are watching very closely and this really is a red line for us - as regards the prospect of Ukraine joining NATO," He told the Ekho Moskvy radio station.
"Of course, this (the question of a membership plan for Ukraine) raises our concerns," he said. Peskov said that Moscow and Washington agreed at the Geneva summit that they needed to holds talks on arms control as soon as possible.
Zelensky said he received assurances that Biden would not use Ukraine as a bargaining tool in his meeting with Russian President Vladimir Putin. He was speaking on the same day as NATO members met for a one-day summit in Brussels.
The Ukrainian president has urged NATO members to accelerate Ukraine's entry into the alliance after a standoff with Russia this year that saw Russia mass additional troops and military equipment near Ukraine's borders.