LONDON/MOSCOW, Dec 13 (Reuters) - Russian President Vladimir Putin told British Prime Minister Boris Johnson on Monday that members of the NATO military alliance were threatening Russia by expanding activity in Ukraine, the Kremlin said. U.S. intelligence assesses that Russia could be planning a multi-front offensive on neighbouring Ukraine as early as next year, involving up to 175,000 troops. The Kremlin denies it plans to invade and says the West is gripped by Russophobia. Moscow says the expansion of NATO threatens Russia and has contravened assurances given to it as the Soviet Union collapsed in 1991. "Vladimir Putin gave a detailed outline of his principal evaluation of the current situation around Ukraine," the Kremlin said of the call with Johnson. "It was underlined that all this is happening against the background of active military "expansion" on the territory of Ukraine by NATO countries creating a direct threat to the security of Russia," the Kremlin said. Johnson warned Putin that any destabilising action against Ukraine would be a strategic mistake with significant consequences, Downing Street said. Downing Street said Johnson "emphasised the UK"s commitment to Ukraine"s territorial integrity and sovereignty, and warned that any destabilising action would be a strategic mistake that would have significant consequences". "He expressed the United Kingdom"s deep concern over the build-up of Russian forces on Ukraine"s border, and reiterated the importance of working through diplomatic channels to deescalate tensions and identify durable solutions."