The London-based newspaper also said Russia had moved a train thought to be linked to a unit of the defence ministry that was responsible for nuclear munitions.Register now for FREE unlimited access to Reuters.comNATO has not observed changes in Russia's nuclear posture, but is vigilant, an alliance official said on Tuesday, while a Western diplomat, commenting on the Times report, told Reuters that NATO had not warned allies about a nuclear threat from Russia.Asked how Britain would respond to the use of "tactical" nuclear weapons by Russia, Britain's foreign minister James Cleverly told a fringe event at the Conservative Party conference that he couldn't go into detail but there would be a response."It would inevitably be the case that the use of nuclear weapons by any country anywhere in the world would not go without a response," he said.Putin on Sept. 21 ordered Russia's first mobilisation of military reservists since World War Two to put more troops on the battlefield and backed a plan to annex swathes of Ukraine, warning the West he was not bluffing when he said he'd be ready to use nuclear weapons to defend Russia."