It is "clear that if Russia uses force against Ukraine, that will have costs, that will have consequences," he told a media conference.

Stoltenberg said a meeting of NATO foreign ministers in Latvia's capital Riga next week would address the massing of Russian military units on the border, which provides "very strong reasons to be deeply concerned".

"This is the second time this year that Russia has massed a large and unusual concentration of forces in the region," he said, listing "heavy capabilities like tanks, artillery armoured units, drones, and electronic warfare systems as well as combat-ready troops".

Stoltenberg called the military buildup "unprovoked and unexplained" and warned "it raises tensions and it risks miscalculations".

Moscow has denounced as Western "hysteria" the concerns that it might be planning to invade Ukraine, part of which -- Crimea -- it annexed in 2014.

The new buildup of Russian forces follows a similar surge in April, when Russia gathered around 100,000 troops on Ukraine's borders.

Stoltenberg said that, although Ukraine is not a member of NATO and did not come under its collective defence pact, the alliance would "send a clear message to Russia that NATO is there to defend and protect all allies".

"We need to make sure that there is no misunderstanding, no room for miscalculation, about NATO's resolve, capability and will to defend all NATO allies," he said.