By Natalia Zinets and Abdelaziz Boumzar KYIV/BAKHMUT, Ukraine (Reuters) -Russian forces tightened their grip on the eastern Ukrainian city of Sievierodonetsk on Monday, a Ukrainian official said, cutting off the last routes for evacuating citizens in a scene that echoed last month’s siege of the port of Mariupol. Amid heavy bombardment, regional governor Sergei Gaidai said on social media that all bridges out of the city had been destroyed, making it impossible to bring in humanitarian cargoes or evacuate citizens. He said 70% of the small industrial city – now the focus of one of the bloodies…