Ukraine says the security situation in rebel-held areas in the east has steadily worsened as separatist fighters move closer to demarcation lines separating them from government forces.

A cease-fire agreed on in September between rebels and Ukraine's government has in effect been rendered invalid as intense hostilities proceed on a daily basis.

National Security and Defense Council spokesman Andriy Lysenko said Thursday that the Russian army is massing troops, including air defense units, near the border.

Ukraine accuses Russia of directly supplying separatist forces, a claim that Moscow has repeatedly denied.

Lysenko's statement follows multiple recent observations of large military convoys on the move around 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 city.