The Russian Foreign Ministry said on Tuesday that the country would have to take "appropriate measures" if Ukraine were to join NATO, according to RIA Novosti.
A Ukrainian government letter to the NATO chief, setting out the country`s bid to join an action plan to gain membership of the Western military alliance, was published in Kiev last week.
"Ukraine`s possible integration into NATO will exacerbate Russian-Ukrainian relations in many areas. We will have to take appropriate measures in response," the ministry said.
Ukrainian Prime Minster Yulia Tymoshenko was quoted by the government`s press service as saying on Saturday that the decision on whether to join NATO would "be taken exclusively by the Ukrainian people through a nationwide referendum." The majority of Ukrainians have so far opposed the idea of joining the alliance.
Meanwhile, the opposition Party of Regions pledged to continue blocking parliamentary work in protest at the government`s move to seek membership of NATO.
Russia has been unnerved by NATO`s eastward expansion and recent U.S. plans to deploy missile defense elements in Poland and the Czech Republic.
In particularly strident language, although not marking a change from earlier stated policies, the Chief of the Russian General Staff, Gen. Yury Baluyevsky, told a conference at the Academy of Military Sciences in Moscow on Saturday that: "the Armed Forces will be used to protect the sovereignty and territorial integrity of Russia and its allies, including preventative action, and including the use of nuclear weapons."
An overwhelming 77% of residents in another former Soviet republic, Georgia, also voted in favor of joining the NATO military alliance at a referendum on January 5.