NATO Secretary-General Jens Stoltenberg says he has told Russian Foreign Minister Sergei Lavrov that Moscow must end its support for the separatists in eastern Ukraine.
Stoltenberg spoke in Brussels on May 19 after holding talks with Lavrov on the sidelines of the annual session of the Council of Europe's Committee of Ministers.
He also told reporters he had told Lavrov that Moscow must be fully transparent about its military exercises in order to avoid potentially dangerous misunderstandings.
"We also discussed the need for transparency, especially when we have an increased military presence along our borders,” he said. “For NATO it is important to do whatever we can to avoid that incidents are spiraling out of control."
Relations between NATO and Moscow have dropped to their lowest point since the Cold War after Russia annexed Ukraine's Crimean Peninsula in March 2014.
The alliance also accuses Russia of backing separatists in Ukraine's east with troops and weapons, which Moscow denies.