North Korea.html">Korea's Worker-Peasant Red Guards conduct a military drill.
The civil defence force group, numbering around 5.7 million, held the exercise against a background of rising hostilities on the Korea.html">Korean peninsula.
South Korean and U.S. troops took part in the joint computer-simulated drill "Key Resolve" on Friday in Seoul.
The annual joint drills started on Monday to practice combined and joint command-post military operations to defend South Korea.
But North Korea is threatening military action against its southern neighbour and the United States if the drills continue.
About 200,000 Korean troops and 10,000 U.S. forces are expected to be mobilized for a separate exercise, "Foal Eagle'', before the end of April.
A spokesman for South Korea's Defence Ministry said on Friday it was keeping an eye on its unpredictable northern neighbour, and that it would be prepared in case of war.
Last week, Pyongyang scrapped the 1953 armistice deal that ended the Korean War and on Monday they cut off the inter-Korean communication hotline.