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Pro-Russian militants stand guard outside an official office in the eastern Ukrainian city of Donetsk on November 13, 2014, as shelling continues in the suburban area of the city near the destroyed airport. MENAHEM KAHANA/AFP/Getty Images

KIEV, Ukraine -- Ukraine warned Thursday that the security situation is steadily worsening in the country's rebel-held east as separatist fighters move closer to government forces. One official said he feared an attack soon by Russian forces.

A cease-fire agreed upon in September between the pro-Russian rebels and Ukraine's government in Kiev has been violated daily, especially around areas coveted by both sides like the airport in the eastern city of Donetsk.

National Security and Defense Council spokesman Andriy Lysenko said the Russian army is massing troops, including air defense units, near the border. Ukraine for months has accused Russia of directly supplying the separatist forces.

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Fighting escalates as Ukraine and NATO claim Russians troops are in eastern Ukriane

Russian Foreign Ministry spokesman Alexander Lukashevich denied the Ukrainian charge.

"There have been and are no military movements across the border or, all the more, any presence of our troops in the southeast of Ukraine," he said Thursday in Moscow.

Ukraine's warnings of possible renewed hostilities follow multiple recent observations of large military convoys on the move around the country's eastern separatist-controlled areas. Trucks transporting troops, ammunition, fuel and large-caliber artillery systems have been seen traveling primarily in the direction of Donetsk, the main rebel-held city.

Ukraine and NATO have said they believe the equipment has been delivered from Russia, although they have yet to provide conclusive evidence for that.

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Ukraine official: Likelihood of imminent Russian attack "high"