NATO warned on Tuesday that Russia has launched a "very serious" military build-up inside Ukraine and on their shared border, deploying troops and sophisticated equipment including air defense systems.

NATO warned on Tuesday that Russia has launched a "very serious" military build-up inside Ukraine and on their shared border, deploying troops and sophisticated equipment including air defense systems.

Troops, artillery and "very modern" air defense weapons could be seen "both inside of Ukraine and also on the Russian side of the border," NATO head Jens Stoltenberg said as he arrived for a meeting with European Union defense ministers.

"It is a very serious build-up," Stoltenberg said, urging Russia to reverse course and meet its commitments under a September ceasefire and peace plan signed by Kiev and separatists.

"Russia has a choice. Russia can either be part of a peaceful negotiated solution or Russia can continue on a path of isolation," Stoltenberg said.

"The international community calls on Russia to be part of the solution."

But for the moment "we see that Russia is still destabilizing Ukraine. We see movement of troops, tanks, artillery, air defense systems in violation of the ceasefire," the former Norwegian premier said.

Stoltenberg took office in October with the Ukraine crisis top of his agenda as the US-led military alliance beefed up its readiness in response to the latest developments in Ukraine.

He is visiting the Baltic states later this week.