"We have heard the signs from Moscow about readiness to continue diplomatic efforts, but so far, we have not seen any de-escalation on the ground," Stoltenberg said Wednesday ahead of a meeting of NATO defence ministers.

"On the contrary, it appears that Russia continues their military build-up," he said.

Stoltenberg said "it remains to be seen whether there is a Russian withdrawal.

"We are of course monitoring very closely what Russia does in and around Ukraine. What we see is that they have increased the number of troops and more troops are on their way."

Russia on Wednesday said military drills in Moscow-annexed Crimea had ended and that soldiers were returning to their garrisons, a day after it announced a troop pullback from Ukraine's borders.

Stoltenberg said that Moscow still maintained the ability to launch a major attack on Ukraine and said that NATO remained "prepared for the worst".

He urged Moscow to carry out a sustained withdrawal of its forces from the border and cautioned that Moscow "always moves forces back and forth".

"If they really start to withdraw forces, that's something we will welcome. But that remains to be seen," he said.

"Just that we see movement of forces, of battle tanks doesn't confirm a real withdrawal."