Ukraine's parliament has voted to drop the country's non-aligned status and work towards Nato membership.
Russian Foreign Minister Sergei Lavrov called the move "counterproductive" and said it would boost tensions.
The BBC's David Stern in Kiev says it is not clear when Ukraine will apply for Nato membership and many officials see it as a distant prospect.
Ukrainian President Petro Poroshenko pledged to seek Nato membership over Russian support for rebels in the east.
Russia, which annexed the Crimean peninsula in March, denies supplying the rebels with weapons. However, it is subject to EU and US sanctions over the crisis.
In a vote in Ukraine's parliament on Tuesday, MPs overwhelmingly backed the move by 303 to eight.
Speaking before the vote, Foreign Minister Pavlo Klimkin said Ukraine was determined to pivot towards Europe and the West.
"This will lead to integration in the European and the Euro-Atlantic space," he said.
The non-aligned status, which Ukraine adopted in 2010 under Russian pressure, prevents states from joining military alliances.
Analysis: Steve Rosenberg, BBC News, Moscow
For the Kremlin, the idea that Ukraine might one day join the European Union is like a bad dream. But the thought of Ukraine joining Nato is a nightmare.
Russia has long complained about Nato "expanding" east, up to Russia's borders. The Kremlin would view Ukraine's membership of the alliance as a direct threat to its national security.
That's unlikely to happen any time soon, especially with the conflict continuing in eastern Ukraine.
But Moscow has been quick to denounce the Ukrainian parliament for renouncing the country's non-aligned status.
Today's vote in the Rada, Ukraine's parliament, certainly drives another wedge between Kiev and Moscow. And it may complicate efforts to resolve the current crisis.
But critics of the Kremlin argue that it is Russia's direct involvement in the conflict in eastern Ukraine which is pushing Kiev closer to Nato.
Addressing foreign ambassadors on Monday night, President Poroshenko said Ukraine's "fight for its independence, territorial integrity and sovereignty has turned into a decisive factor in our relations with the world".
Russia has made clear that it opposes Ukraine's move towards Nato.
Andrei Kelin, Russia's envoy to the Organisation for Security and Co-operation in Europe (OSCE), said on Tuesday: "It's an unfriendly step towards us. This political vector will only add to nuisances and acuteness in ties."
In a Facebook post on Monday, Russian Prime Minister Dmitry Medvedev warned that Ukraine's rejection of neutrality would have "negative consequences".
"In essence, an application for Nato membership will turn Ukraine into a potential military opponent for Russia," he wrote.
A Nato spokesman in Brussels said on Tuesday that any accession to the alliance would probably take years, Reuters reported.
Some Nato members are also lukewarm towards accepting Ukraine, says the BBC's David Stern in Kiev.
The roots of the current conflict go back to Ukraine's pro-EU street protests at the end of last year that led to the fall of Russian-leaning President Viktor Yanukovych in February.
After Crimea was annexed, pro-Russian separatists seized parts of the Donetsk and Luhansk regions of eastern Ukraine and later declared independence.
Since then, nearly 5,000 people have died and another million have been displaced by fighting.
A ceasefire was signed by both sides in Minsk in September but observers say some fighting is continuing.