By Nandita Bose WASHINGTON (Reuters) -U.S. President Joe Biden on Monday accused Russian President Vladimir Putin of war crimes and called for a trial, adding to the global outcry over civilian killings in the Ukrainian town of Bucha as more graphic images of their deaths emerged. “You saw what happened in Bucha,” Biden told reporters after landing in Washington from Delaware, where he had spent the weekend. “This warrants him – he is a war criminal.” The discovery of a mass grave and tied bodies shot at close range in Bucha, outside Kyiv, a town Ukrainian forces reclaimed from Russian troops,…