LONDON, Oct 24 (Reuters) - Russia's Defence Ministry said on Monday that it had prepared its forces to work in conditions of radioactive contamination, after Moscow accused Ukraine of planning to detonate a "dirty bomb" - something Kyiv has strongly denied.After weeks of rising international tension following threats by President Vladimir Putin to defend Russia's "territorial integrity" with nuclear weapons, it was the first concrete statement from Moscow of a change in its forces' state of preparedness.Defence Minister Sergei Shoigu told Western defence ministers on Sunday that Moscow believed Ukraine was preparing to detonate such a bomb - a device using conventional explosives packed with radioactive material to spread contamination over a wide area.Ukraine's Western allies have dismissed any suggestion that Kyiv is building or planning to use a dirty bomb, and Ukrainian President Volodymyr Zelenskiy said the accusation was a sign that Moscow was planning such an attack itself and would blame Kyiv."The unfounded denials of our Western colleagues, saying all this is fiction and that Russia itself plans to do something similar in order to later blame the Zelenskiy regime - this is not a serious conversation," he told reporters after a meeting of the Valdai International Discussion Club."