By Jarrett Renshaw and Gabriela Baczynska BRUSSELS (Reuters) -NATO promised Kyiv new military support and assigned more troops to the alliance’s eastern flank while London and Washington stepped up sanctions on Moscow during a trio of summits on Thursday aimed at showing Western unity against Russia’s war in Ukraine. NATO leaders meeting in Brussels agreed to help Ukraine protect itself against any chemical, biological or nuclear attacks, and a U.S. official said allies were working to provide Kyiv with anti-ship missiles. “The single most important thing is for us to stay unified and the worl…