Russia faces growing allegations of war crimes after dozens of Ukrainian prisoners of war were massacred in a strike that both countries are blaming on each other. Russia’s military says Ukraine used long-range HIMARS missiles recently provided by the U.S. to hit a prison camp in Russian-held territories in Eastern Ukraine. President Zelenskyy called it a false flag attack by Russia and an act of terrorism, imploring the world to act. Among those killed were the last standing Ukrainian fighters in the Azovstal Steel Plant, according to the Russian Defence Ministry. The U.N. is now calling for an investigation. » Subscribe to NBC News: http://nbcnews.to/SubscribeToNBC » Watch more NBC video: http://bit.ly/MoreNBCNews NBC News Digital is a collection of innovative and powerful news brands that deliver compelling, diverse and engaging news stories. NBC News Digital features NBCNews.com, MSNBC.com, TODAY.com, Nightly News, Meet the Press, Dateline, and the existing apps and digital extensions of these respective properties. We deliver the best in breaking news, live video coverage, original journalism and segments from your favorite NBC News Shows. Connect with NBC News Online! NBC News App: https://smart.link/5d0cd9df61b80 Breaking News Alerts: https://link.nbcnews.com/join/5cj/breaking-news-signup?cid=sm_npd_nn_yt_bn-clip_190621 Visit NBCNews.Com: http://nbcnews.to/ReadNBC Find NBC News on Facebook: http://nbcnews.to/LikeNBC Follow NBC News on Twitter: http://nbcnews.to/FollowNBC Follow NBC News on Instagram: http://nbcnews.to/InstaNBC At Least 50 Ukrainian Prisoners Of War Killed In Strike On Prison Detention Center