"There are signs from Moscow that diplomacy should continue, this gives grounds for cautious optimism. But so far, we have not seen any sign of de-escalation on the ground," Stoltenberg told journalists.

Russia said Tuesday it was pulling back some of its forces near the Ukrainian border to their bases, after suggesting a diplomatic solution to its standoff with the West was still possible.

Stoltenberg warned that Moscow still had "everything in place" to stage an attack on Ukraine at any time, but said the Kremlin "has time to step back from the brink".

"We will continue to monitor and to follow closely what Russia is doing," Stoltenberg said ahead of a meeting of NATO defence ministers Wednesday.

The US ambassador to NATO, Julianne Smith, said it was too early to say if Moscow was in fact withdrawing its forces.

"Obviously, we would have to see much of that force posture shift for us to feel reassured about de-escalation," she said.

"We will work to try and verify and go from there."

Tensions have soared over Ukraine as the West warned that over 100,000 Russian troops deployed to its border could be about to invade.

"We have systematically exposed Russia's actions, plans and disinformation, to lay bare to the world what Russia is doing, and to make it harder for Russia to conduct aggressive actions," Stoltenberg said.

Stoltenberg said NATO wanted to see Moscow withdraw not only troops but also military hardware that it has parked along the frontier, viewing that as proof of an enduring de-escalation.

At the same time as it has ramped up pressure on Ukraine, Moscow has issued security demands that the West has rejected as non-starters, including barring Ukraine from joining the alliance and rolling back forces from eastern Europe.

NATO is waiting for Russia to respond to an offer to further talks on a raft of counter-proposals covering transparency on exercises and arms control.

The crisis over Ukraine has helped to revitalise the US-led alliance after its catastrophic departure from Afghanistan last year.

Western allies, spearheaded by Washington, have sent thousands of new troops to bolster NATO's eastern flank and are looking at establishing new battle groups in the region.