BRUSSELS (AFP) - Russia is making a "very serious" military build-up in Ukraine and on their shared border, deploying troops and sophisticated equipment including air defence systems, Nato head Jens Stoltenberg said on Tuesday.

Moscow could be "part of a peaceful negotiated solution or Russia can continue on a path of isolation," Stoltenberg said, urging it to meet its commitments to a September peace plan signed by Kiev and pro-Kremlin rebels.