The vote by Ukraine's parliament to drop its nonaligned status, which could pave the way for a bid to join NATO, challenges the Kremlin's ardent desire to keep NATO from taking a giant step toward the Russian heartland.
The move is likely to add difficulties to Wednesday's round of talks aimed at resolving the Ukrainian crisis.
Five NATO countries — Norway, Estonia, Latvia, Lithuania and Poland — now share relatively short borders on Russia's western outskirts, totaling about 1,300 kilometers (780 miles). Adding Ukraine's 1,500-kilometer (900-mile) border with Russia to that would move the alliance's eastward flank substantially, and put it roughly on the same longitude as Moscow.
Supporters of the Tuesday move, which passed by a 303-9 vote, said it was justified by Russian aggression toward Ukraine, including the annexation of its Crimean Peninsula in March and Russian support for a separatist insurgency in eastern Ukraine, where some 4,700 people have been killed since the spring.
But opponents said it will only increase tensions, and Moscow echoed that view.
"This is counterproductive, it only heats up the confrontation, creating the illusion that accepting such a law is the road to regulating the deep internal crisis in Ukraine," said Russian Foreign Minister Sergey Lavrov.
Russia routinely characterizes the Ukrainian crisis as an internal matter and rejects claims from Ukraine and the West that it has sent troops and equipment to rebels in eastern Ukraine and shelled government positions from Russian border areas.
Although Ukraine had pursued NATO membership several years ago, it declared itself a nonbloc country after Russia-friendly Viktor Yanukovych became president in 2010. Yanukovych fled the country in February after months of street protests that exploded into violence, and was replaced by Western-leaning Petro Poroshenko in May.
Russia's annexation of the Crimean Peninsula and its support for the separatist insurgency appear partly rooted in fears that the Western military alliance could expand its presence on the Russian border.
The vote does not mean that Ukraine will apply to join NATO. But "in the conditions of the current aggression against Ukraine, this law opens for us new mechanisms," Ukrainian Foreign Minister Pavlo Klimkin told the parliament.
However, Ukraine's prospects for NATO membership in the near term appear dim. With its long-underfunded military suffering from the war with the separatists and the country's economy in peril, Ukraine has much to overcome to achieve the stability that the alliance seeks in members.
An alliance official, who spoke on condition of anonymity in keeping with NATO practice, told The Associated Press "our door is open and Ukraine will become a member of NATO if it so requests and fulfils the standards and adheres to the necessary principles."
Negotiators from Ukraine, Russia, the Organization for Security and Cooperation in Europe and the eastern rebels are to meet Wednesday in the Belarusian capital Minsk for another round of talks on resolving the Ukraine crisis.
Since a cease-fire agreement was reached in Minsk on September 5, fighting has diminished. But both sides report frequent cease-fire violations and there is incomplete progress on the stipulation for each side to pull back its heavy weaponry to create a buffer zone.
Russia's envoy to NATO, Alexander Grushko, told the Interfax news agency that the Ukrainian vote creates "serious complications in the search for a way to end the violence and change the situation into a political process."
Another meeting of the so-called "contact group" in Minsk has been set for Friday, indicating no significant decisions are expected at the Wednesday session.
Heidi Tagliavini of Switzerland, the OSCE's lead figure at the talks, said the participants would discuss how to solidify the cease-fire, the pullback of heavy weaponry, a complete exchange of prisoners taken by both sides in the conflict and the delivery of humanitarian aid.
The volatile issue of determining a final status for the separatist regions is far from resolved. The rebels had sought absorption into Russia, but Moscow has fended off that request, instead calling for a federalization of Ukraine that would give the country's regions more autonomy from the central government.
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