A Defence Ministry official said that North Korea fired two missiles, estimated to be Scud ballistic missiles, between 6:32 a.m. and 6:41 a.m. from Nampo areas, South Pyongan province, said a report from Yonhap News Agency, according to Xinhua.
Those missiles, launched from Nampo, North Korea's western coast city located southwest of capital Pyongyang, flied about 490 km across its territory into eastern waters.
The official said the South Korean military was closely watching possible provocations, including additional launch of missiles.
The General Staff Department of the Korean People's Army of North Korea said in a statement earlier that such joint war games were rehearsals for the northward nuclear war to remove North Korean leadership and occupy Pyongyang.
Seoul and Washington launched their joint annual war games on Monday. The "Key Resolve" command post exercise will be held from March 2 to March 13, mobilising about 10,000 South Korean troops and 8,600 US forces.
The "Foal Eagle" filed training exercise will be conducted from March 2 to April 24, involving about 200,000 South Korean troops and 3,700 American soldiers, including 3,500 US forces from overseas.
The "Foal Eagle" is a series of combined field maneuver exercise spanning ground, air, naval and special operations.