NATO says there's a risk of a return to heavy fighting this summer in eastern Ukraine, and is blaming Russia.

NATO Secretary-General Jens Stoltenberg said Thursday Russia continues to support pro-Moscow separatists "with training, weapons and soldiers."

He said the Russian military also has stationed "large numbers of forces" on the border with Ukraine.

Russia denies any involvement in the conflict that Stoltenberg said has already cost more than 6,000 lives.

NATO defense ministers are meeting in Brussels with their Ukrainian counterpart, Stepan Poltorak, to discuss the latest developments and NATO support for Ukraine.

NATO has ruled out supplying weapons to Ukraine, but is helping in other ways. Stoltenberg said there had been recent progress in the areas of helping Ukraine secure its communications and its airspace.