NATO's top military commander Philip Breedlove said in Bulgaria the alliance had spotted military equipment arriving from Russia in regions of east Ukraine that are held by pro-Russian separatist rebels.

General-Major Igor Konashenkov, a Russian Defense Ministry official, dismissed the comments as anti-Russian "hot air" and said "there was and is no evidence" behind the NATO accusations.

"We have already stopped paying attention to unsubstantiated statements by NATO's Supreme Allied Commander Europe, General Philip Breedlove, on Russian military convoys he 'observed' allegedly invading Ukraine," Konashenkov said.

The West has imposed sanctions on Russia over the crisis in Ukraine, saying Moscow has provided the rebels with arms and reinforced them with troops since they rose up a month Russia annexed the Crimea peninsula from Ukraine in March.

Breedlove said in Bulgaria that NATO had seen "Russian equipment, primarily Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops entering into Ukraine" in recent days.

Fighting has intensified around the rebel stronghold of Donetsk in east Ukraine, putting further pressure on a patchy cease-fire between rivals. Kiev says it is redeploying forces to prepare for a possible new rebel offensive.