Russia is again warning Ukraine that any move to join NATO will "seriously" complicate bilateral ties and force Moscow to take "appropriate action."
The warning was released Tuesday in a Russian Foreign Ministry statement.
Last week, Western-leaning Ukrainian President Viktor Yushchenko and Prime Minister Yulia Tymoshenko formally asked the military alliance for advice and support that could lead to Kyiv's eventual membership.
Ukrainian access to NATO's Membership Action Plan does not guarantee Kyiv NATO membership, and Ukrainian officials have in the past pledged to hold a voter referendum before moving to join the alliance.
Russian President Vladimir Putin, last year, called NATO's expansion to Russia's borders harmful, and said it does not foster an atmosphere of trust.
He also noted that NATO was founded to counter the Soviet Union, which no longer exists.
NATO's last expansion in 2004 included Estonia, Latvia and Lithuania, bringing the military alliance to Russia's western border.
Some information for this report was provided by AFP.