Rebels have downed Ukrainian planes flying at high altitudes . The Russian Defense Ministry said the incident was a "provocation" and that it would be dealt with firmly. The border region with Russia is very porous , unguarded in many places . It added , notably , that `` more advanced air defense systems have also arrived at this site . '' Some contend that larger weapons have come into Ukraine from Russia .
Rebels have downed Ukrainian planes flying at high altitudes . The Russian Defense Ministry said the incident was a "provocation" and that it would be dealt with firmly. The border region with Russia is very porous , unguarded in many places . Some contend that larger weapons have come into Ukraine from Russia .