Ukrainian soldiers drive on an airborne combat vehicle near Kramatorsk, in eastern Ukraine April 16, 2014. Ukrainian forces tightened their grip on the eastern town of Kramatorsk on Wednesday after securing control over an airfield from pro-Russian separatist militiamen, prompting Russian President Vladimir Putin to warn of the risk of civil war. REUTERS/Marko Djurica (UKRAINE - Tags: POLITICS CIVIL UNREST MILITARY)