A pro-Russian rebel aims his rifle at a checkpoint near a Ukrainian airbase in Kramatorsk, eastern Ukraine May 2, 2014. Pro-Russian rebels shot down two Ukrainian helicopters on Friday, killing two crew, as troops tightened their siege of separatist-held Slaviansk in what Moscow called a "criminal" assault by Kiev that wrecked hopes of peace. REUTERS/Marko Djurica (UKRAINE - Tags: POLITICS CIVIL UNREST)