Pyongyang has long coveted an advanced air-defense system to guard against missiles and war planes from the United States and South Korea.

A top South Korean official says Russia has supplied air defense missile systems to North Korea in exchange for Pyongyang sending troops.

North Korea condemned on Saturday recent joint military drills by the United States, South Korea and Japan, warning that it would take immediate actions if needed to defend the state. Last week, the three countries held a three-day joint exercise,