Russia has begun massing troops near its border with Ukraine again, the North Atlantic Treaty Organization said, in what would mark a new escalation of the crisis there as the West revived its threat of more sanctions.

Russian President Vladimir Putin appeared to dig in, blaming Kiev for the deepening turmoil. The Kremlin said he told the leaders of Germany and France on Thursday that he was concerned about "the continuing military operation in eastern Ukraine," where fighting has been raging for days.

The statement from Moscow made no mention of alleged troop movements or the threat of further sanctions.

A renewed buildup along the border—weeks after Russia withdrew most of what NATO had said were 40,000 troops positioned there—would mark a setback for Western diplomats. They have been pushing Moscow to support a proposal from Ukraine's new President Petro Poroshenko for a cease-fire with pro-Russia rebels ahead of disarmament and a national dialogue.

Anders Fogh Rasmussen, NATO's secretary-general, said Thursday in London that NATO saw "at least a few thousand more Russian troops" deployed to the Ukrainian border as well as troop maneuvers nearby.