(Reuters) – The United States warned of “catastrophic consequences” if Moscow were to use nuclear weapons in Ukraine, after Russia’s Foreign Minister said regions holding widely-criticised referendums would get full protection if annexed by Moscow. Votes in four eastern Ukrainian regions, aimed at annexing territory Russia has taken by force mostly since its invasion in February, were staged for a third day on Sunday. The Russian parliament could move to formalise the annexation within days. By incorporating the four areas of Luhansk, Donetsk, Kherson and Zaporizhzhia into Russia, Moscow could…