KYIV, Ukraine (AP) – Russia’s defense chief has alleged that Ukraine was preparing a “provocation” involving a radioactive device, a stark claim that was strongly rejected by U.S., Ukrainian and British officials amid soaring tensions as Moscow struggles to stem Ukrainian advances. Russia’s Defense Minister Sergei Shoigu made the allegations in phone calls Sunday with his counterparts from the United States, Britain, France and Turkey. The Russian Defense Ministry said Shoigu voiced concern about possible Ukrainian provocations involving a “dirty bomb,” a device that uses explosives to scatter radioactive waste. Russia repeatedly has made allegations that Ukraine could detonate a dirty bomb and blame it on Moscow. Ukrainian authorities, in turn, have accused the Kremlin of hatching such a plan.