By Guy Faulconbridge and Jonathan Landay LONDON/ZAPORIZHZHIA, Ukraine (Reuters) -An ally of President Vladimir Putin issued a stark new nuclear warning to Ukraine and the West on Tuesday as Russia began releasing results of referendums it bills as a prelude to it annexing four Ukrainian regions. But an adviser to Ukrainian President Volodymyr Zelenskiy told Reuters Kyiv would not be swayed by nuclear threats or by the annexation votes, and would press on with plans to retake all territory occupied by invading Russian forces. Europe was investigating on Tuesday what Germany and Denmark said wer…