A Ukrainian serviceman digs trenches at his position in Avdeyevka near Donetsk, Ukraine, June 4, 2015. Ukraine's president Petro Poroshenko told his military on Thursday to be prepared for the possibility of a "full-scale invasion" by Russia across the entire length of the joint border, a day after the worst fighting with Russian-backed separatists in months. REUTERS/Oleksandr Klymenko