* Assad's forces and Kurds coexisted during war * Clashes erupt in Syria's mainly Kurdish **rtheast * Army shells Kurdish-majority areas near Hassakeh city DIYARBAKIR, TURKEY/BEIRUT, Jan 17 (Reuters) - Syrian Kurds battled on Saturday with forces loyal to President Bashar al-Assad, Kurdish sources and a monitoring group said, breaking a longstanding tacit agreement between the two sides to focus on other enemies in a complex civil war. In Syria's predominantly Kurdish **rtheast, Assad's forces and Kurdish militia, mainly the Syrian Kurdish People's Protection Units (YPG), have for the most part coexisted without clashing, focusing their firepower on the Islamic State insurgent group.