NATO head Jens Stoltenberg urged Russia on Tuesday for full transparency during snap military exercises in order to avoid potentially catastrophic misunderstandings as the Ukraine crisis stokes tensions.

Transparency is paramount, Stoltenberg said after talks with Russian Foreign Minister Sergei Lavrov on the sidelines of a Council of Europe meeting in Brussels.

"For NATO it is important to do whatever we can to avoid that the incidents and the situation is spiralling out of control," he said.

"The snap exercises, the numbers ... reduce predictability and transparency along our borders and I think we should try to change that by the Russians being more transparent when it comes to their military activities."

NATO was "fully transparent when it comes to when and where we are going to conduct exercises," he added.

Stoltenberg's meeting with Lavrov was their first since early February and comes amid signs that tensions over Ukraine may be easing slightly although a tenuous ceasefire between Kiev forces and pro-Russian rebels continues to be breached daily.

Stoltenberg said they had discussed the "very serious situation in eastern Ukraine" and repeated that Moscow must do its part to ensure full implementation of the Minsk ceasefire accords, including withdrawal of heavy weapons and full access for OSCE monitors.

Russia should also "withdraw all its troops and support for the separatists," he added.

Moscow rejects claims that its troops are directly involved in a conflict which has cost more than 6,100 lives since April last year but makes clear it backs the rebels in eastern Ukraine.

NATO cut all practical cooperation with Russia that month after Moscow annexed Crimea, but Stoltenberg stressed that the US-led military alliance kept its lines of communication open.

"It is important to have these lines of communication and they can contribute to more predictability and avoiding any misunderstandings."

"We are keeping the lines of political contact open and we will continue to do so," he added.