By Pavel Polityuk and Phil Stewart KYIV (Reuters) -President Volodymyr Zelenskiy vowed there would be no let-up in Ukraine’s fight to regain its territory as Kyiv said its troops had crossed a major river, paving the way for an assault on Russia’s occupation forces in the eastern Donbas region. Reflecting the dramatic change in momentum since Ukrainian forces routed Russian troops earlier this month in the northeast, U.S. President Joe Biden offered his strongest prediction so far that the Ukraine would win the war. “They’re defeating Russia,” Biden said in an interview with CBS’s “60 Minutes”…