The annual "Ulchi Freedom Guardian" exercise involves about 530,000 troops from the RoK, the US and seven other countries, as well as computer-aided simulations, the US command in Seoul reported.

The joint UN command has notified its DPRK counterparts of the exercise, US military officials announced.

No peace treaty has formally ended the 1950-1953 Korean War, only an armistice that has left the peninsula split by the world's most heavily fortified border.

The secretive, economically stunted the DPRK tested nuclear weapons in 2006 and 2009, while sporadic artillery duels and sea clashes still rattle the region.

Last week, the RoK fired at DPRK after it said northern gunners fired three shots close to its maritime border.

No casualties were reported.