The government in Kiev is amassing Ukrainian military units and special forces in the country's eastern region as part of an anti-terror operation against pro-Russia separatists, NBC News reported. Ukraine's interim president, Oleksander Turchinov, said on Tuesday that Ukrainian forces had retaken control of the Karamatorsk airfield. Heavy gunfire was heard in the area, although no deaths have been confirmed. Separatists continue to defy orders by Turchinov to leave the country's east or surrender, sparking fears that violent clashes between the two sides could escalate. Russian President Vladimir Putin told U.N. Secretary-General Ban Ki-moon in a phone call on Tuesday that he expects the U.N. to condemn Ukraine's use of force, calling its actions "anti-constitutional."