"They tried again to launch an offensive this spring and summer -- but so far only marginal gains," Stoltenberg said in an interview with AFP.

"And we don't have any other indications or reason to believe that Russia has the capabilities, the strength to make big breakthroughs."

The head of the Western military alliance said that Russia would nevertheless most likely "continue to push on the front lines" and carry out airstrikes.

"But what we have seen is that the Ukrainians have been able to hold the line," he said.

"They have been able to continue to inflict heavy losses on the Russian invaders, both along the front line, but also by deep strikes."

Russia has been on the front foot against Ukraine's outgunned forces after launching a renewed offensive in the Kharkiv region in May.

NATO countries, spearheaded by the United States, have managed to ramp up deliveries of weaponry to Kyiv again after Washington passed a stalled $60-billion aid package in April.