In Ukraine the defence ministry said a cyber attack was into its second day. Russia said it had nothing to do with that.

The Russian defence ministry said its forces were pulling back after completing exercises in the southern and western military districts near Ukraine.

It published video that it said showed tanks, infantry fighting vehicles and self-propelled artillery units leaving the Crimean peninsula, which Moscow seized from Ukraine in 2014.

NATO Secretary-General Jens Stoltenberg said a pullout would be welcome but that moving troops about did not confirm it.

"It remains to be seen whether there is a Russian withdrawal ... What we see is that they have increased the number of troops, and more troops are on the way," he told reporters at the start of a two-day meeting of NATO defence ministers at the alliance's headquarters in Brussels.

The Kremlin said NATO's assessment was wrong. Moscow's ambassador to Ireland said forces in western Russia would be back to their normal positions within three to four weeks.

The deployment in the Crimean peninsula was part of a huge build-up of Russian forces to the north, east and south of Ukraine since November that had prompted London and Washington to warn in recent days that a Russian invasion looked imminent.

Russia mocked those warnings as hysterical war propaganda when it announced on Tuesday that some units were starting to return to base after completing exercises.

China, which has cultivated closer ties with Russia as both countries have come under increasing criticism from the West, accused the United States of "playing up the threat of warfare and creating tension".

Military analysts say a key indicator of a significant pullback will be whether field hospitals and fuel stores are dismantled and units from Russia's far east, which are taking part in huge exercises in Belarus this week, return to their bases thousands of kilometres away.

US President Joe Biden said on Tuesday that more than 150,000 Russian troops were still amassed near Ukraine's borders and an invasion remained "distinctly possible". He said Washington had not yet verified any pullout.

Biden has warned repeatedly of steep costs for Russia if it attacks Ukraine, including sanctions against Russian businesses and the Nord Stream 2 gas pipeline from Russia to Germany.

But he also reaffirmed willingness to talk to Russia about arms control, transparency measures and strategic stability.

Kremlin spokesman Dmitry Peskov said President Vladimir Putin was also keen to talk.

"We hope that this negotiation process will begin," he said, adding that it would be complex and require flexibility on both sides.

Russia has always denied planning to invade Ukraine but has been pressing for a set of security guarantees from the West including a promise that its neighbour Ukraine will never join NATO. The United States and its allies have rejected that.

The defence ministry said hackers were still bombarding its website and had succeeded in finding vulnerabilities in the programming code. Traffic was being rerouted to servers in the US while the issue was being fixed, it said.

The Kremlin denied Russia was behind any cyber attacks.

Kremlin spokesman Peskov said Putin had "taken note" of a request from Russia's parliament on Tuesday for him to recognise the "independence" of two breakaway regions in eastern Ukraine where Russian-backed separatists have been fighting Ukrainian government forces since 2014.

But he said that would not be line with the 2014-15 Minsk agreements aimed at ending the conflict, in which Ukraine says some 15,000 people have been killed.

Australian Associated Press