Britain's foreign minister James Cleverly on Tuesday said Russian President Vladimir Putin's sequence of strategic errors must stop and that any use of nuclear weapons would lead to consequences.
Putin has escalated its seven-month war in Ukraine with a military mobilisation and warnings of nuclear weapons use.
Asked how Britain would respond to the use of tactical nuclear weapons by Russia, 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.
"I'm not going to discuss the nature or the threshold."
Earlier Kremlin spokesman Dmitry Peskov said Moscow did not want to take part in "nuclear rhetoric" spread by the West, after Britain's Times newspaper reported that NATO had warned members Putin might test an atomic weapon on Ukraine's border.
Cleverly said strategic errors by Putin had damaged Russia and left him increasingly isolated globally.
"We need to make it very clear that his sequence of strategic errors has got to stop," he said. "We will continue to support Ukrainians in the defence of their homeland, and stand up for the international rules and norms".
(Reporting by Alistair Smout; editing by William James)
