Moscow - Nato chief Jens Stoltenberg criticised Russia on Tuesday for its decision to add at least 40 new intercontinental ballistic missiles to its nuclear arsenal this year - a move that Moscow has blamed on the military alliance."This nuclear sabre-rattling of Russia is unjustified, it's destabilising, and it's dangerous," Stoltenberg said in Brussels.Tensions between Nato and Russia have reached a post-Cold War peak over the crisis in Ukraine. The West accuses Moscow of fomenting the conflict, which Russia has steadfastly denied.Nato beefed up the eastern flank of the alliance, after member states in eastern Europe said they felt threatened by Russia's actions in Ukraine."We had to respond because we see a Russia which is more assertive and which is responsible for aggressive actions," Stoltenberg said, arguing that everything Nato does is "fully in line with our international commitments and obligations".The United States over the weekend revealed plans to station tanks and fighter jets in the Baltic countries as further reassurance measures.Russian Deputy Defence Minister Anatoly Antonov said in comments carried by state media that Nato was dragging his country into a new arms race.Russia has vowed to retaliate against any military build-up on its borders, and on Tuesday President Vladimir Putin announced the missile plan.The nuclear missiles will be "capable of overcoming even the most technically advanced missile-defence systems", he said at a military industrial forum in the Moscow region, the Interfax news agency reported.'Nobody wants to see us step backwards'US Secretary of State John Kerry said he had "concerns" about Putin's missile announcement, citing agreements under which the former Cold War adversaries both sharply reduced nuclear stockpiles."We have the START agreement - we're trying to move in the opposite direction," he said. "We've had enormous co-operation from the 1990s forward with respect to the destruction of nuclear weapons that were in former territories of the Soviet Union."He said the Russian moves "could well be posturing" in response to Nato's actions to reassure the alliance's own frontier states."Nobody wants to see us step backwards. Nobody wants to, I think, go back to a kind of Cold War status," Kerry said."It's really hard to tell [Putin's motives], but nobody should hear that kind of announcement from the leader of a powerful country and not be concerned about what the implications are."