NATO's secretary-general warned of "grave consequences" for the alliance's relations with Moscow if Russia encroaches further into eastern Ukraine, where demonstrators have seized several public buildings.
Anders Fogh Rasmussen on Tuesday said that NATO was watching developments in southern and eastern Ukraine closely and urged Moscow to "step back and not escalate the situation."
"Russia's illegal aggression against Ukraine is the greatest challenge to European security in a generation," Rasmussen said in Paris.
Pro-Russian demonstrators in three cities - Kharkiv, Donetsk and Luhansk - seized state buildings in recent days, calling for regional independence.
Ukrainian authorities have reclaimed the buildings in Kharkiv, saying they arrested 70 people in the process.
The interim government in Kyiv has accused Russia of orchestrating the unrest, US government spokesman Jay Carney also said there was "strong evidence" that some of the protesters "were paid and were not local residents."
Rasmussen issued his warning to Moscow without explicitly mentioning Russia's alleged involvement in the latest unrest.
"If Russia were to intervene further in Ukraine it would be a historic mistake," Rasmussen said. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."
He called for Russia to pull back the "tens of thousands" of troops he said were stationed near Ukraine's eastern border, also asking Russia to enter dialogue with Kyiv's new government and to "respect its international commitments."
Western powers and the government in Kyiv have voiced concern that further parts of Ukraine could face a similar fate to the southern peninsula of Crimea, which last month voted to join the Russian Federation in a contentious referendum.
Russia's Foreign Minister Sergei Lavrov dismissed accusations that Russia was behind the protests, saying after a meeting in Moscow with his Angolan counterpart: "One should not seek to put the blame on someone else."
As for claims of Moscow funding the protests, Lavrov said he would "leave these statements on the conscience of our American partners."
Anders Fogh Rasmussen will leave his post as NATO's secretary-general later this year, former Norwegian premier Jens Stoltenberg will replace him.
msh/ipj (AFP, dpa, Reuters)