A Ukrainian soldier stands guard near an armoured personnel carrier at a checkpoint near the town of Slaviansk in eastern Ukraine

A Ukrainian soldier stands guard near an armoured personnel carrier at a checkpoint near the town of Slaviansk in 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 and Moscow accused Kiev of launching a "criminal" assault that wrecked hopes of peace.  REUTERS/Baz Ratner (UKRAINE - Tags: POLITICS CIVIL UNREST MILITARY) - RTR3NJH2