Brussels: Russia has launched a "very serious" military build-up inside Ukraine and on their shared border, deploying troops and sophisticated equipment including air defence systems, Nato head Jens Stoltenberg said.
Troops, artillery and "very modern" air defence weapons could be seen "both inside of Ukraine and also on the Russian side of the border," Stoltenberg said as he arrived for a meeting with European Union defence 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 pro-Kremlin rebels.
"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 defence systems in violation of the ceasefire," the former Norwegian premier said.
Air defence systems are a particular concern after the shooting down of Malaysia Airlines flight MH17 over eastern Ukraine in July, which western governments allege was carried out by pro-Kremlin rebels.
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 Russia's actions which spooked newer Nato members such as Poland and the Baltic states once ruled from Moscow.
He is visiting the Baltic states later this week.
Nato has deployed aircraft and personnel on a rotational basis through its eastern member states in an effort to reassure them and embarked on longer term planning in the face of what it sees as a more assertive Moscow under President Vladimir Putin.
Putin insists Russia has no presence in eastern Ukraine where the rebels appear to be consolidating areas under their control.