By Idrees Ali and Phil Stewart WASHINGTON (Reuters) -The United States assesses that Russia is prioritizing eastern Ukraine’s Donbass, as opposed to Kyiv, in what could be an effort to build leverage in negotiations and cut off Ukrainian forces in the east from the rest of the country, a senior U.S. defense official said on Friday. Russia said earlier on Friday that the first phase of its military operation was mostly complete and it would focus on completely “liberating” Ukraine’s breakaway eastern Donbass region. The announcement appeared to indicate Moscow may be switching to more limited o…