To hold what remained of their territory , the pro-Russian rebels needed to be able to challenge Ukrainian dominance of the skies . Rebels have downed Ukrainian aircraft flying at high altitudes . A pro - Russian rebel commander set off anti - aircraft missiles . The border area with Russia is very porous , unguarded in many places . Some believe that larger weapons have come into Ukraine from Russia .
Rebels have downed Ukrainian aircraft flying at high altitudes . A pro - Russian rebel commander set off anti - aircraft missiles . The border area with Russia is very porous , unguarded in many places . Some believe that larger weapons have come into Ukraine from Russia .