"Ceasefire violations persist. There is still a risk of a return to heavy fighting," Stoltenberg said in Brussels on Thursday, AFP reported.

"Russia continues to support the separatists with training, weapons and soldiers. It has a large number of forces stationed on its border with Ukraine. The best chance for peace is the full implementation of the Minsk agreements. I urge all parties to do so and Russia has a special responsibility in this regard," said Stoltenberg.

NATO remained committed to "Ukraine's territorial integrity and sovereignty," and would not recognize Russia's "illegal and illegitimate annexation of Crimea," he added, according to the agency, which noted it its report that Russia "denies that it is directly involved in the 15-month conflict in Ukraine."