NATO on Monday said Russia had increased its troops near the Ukraine border to as many as 12,000 after reducing them to less than 1,000 in June, hurting efforts to ease the crisis.

"This is not a step in the right direction. It is a step away from de-escalating the situation," a NATO official said.

"Our current assessment is that between 10,000 and 12,000 Russian troops are in the area."

The official said NATO urged "Russia once again to pull back its troops," stop supporting pro-Kremlin rebels and halt the flow of arms and funding.

"This activity is highly destabilising and has to stop," the official said, as Kiev charged that a Ukrainian military plane downed over rebel territory was "likely" shot from Russia.

Tensions on the border soared after a shell reportedly from the Ukrainian side killed a Russian civilian on Sunday.

As the Ukraine crisis deepened with Moscow's annexation of its Crimea territory in March, Russia massed some 40,000 troops on the border and ignored repeated calls from the West for their withdrawal as part of efforts to resolve the crisis.

However in May, Russian President Vladimir Putin announced an end to military exercises in the area and the return to bases of the troops involved as diplomatic efforts to solve the conflict intensified.