Tens of thousands of South Korean and US troops have started a large-scale military exercise simulating an all-out attack by North Korea, which has condemned the joint drill as a "declaration of war".
The annual Ulchi Freedom exercise, which the defence ministry said would run through to August 28, is largely computer-simulated, but still involves 50,000 Korean and 3000 US soldiers.
The drill plays out a full-scale invasion scenario by North Korea, and both Seoul and Washington insist it remains purely defensive in nature.
Pyongyang views Ulchi Freedom - along with other annual South Korea-US drills - as wilfully provocative and has threatened the "strongest military counter-action" should this year's exercise go ahead.
"Such large-scale joint military exercises ... are little short of a declaration of a war," the North's Committee for the Peaceful Reunification of Korea, which oversees cross-border issues, said last week.
The committee specifically warned of the drill's potential for an accidental military clash that could trigger an "all-out" conflict.
Military tensions are already running high along the Korean peninsula after South Korea blamed the North for mine blasts that maimed members of a border patrol earlier this month.
The South retaliated by resuming high-decibel propaganda broadcasts across the border, using batteries of loudspeakers that had lain silent for more than a decade.
North Korea has denied any involvement and, at the weekend, threatened "indiscriminate" strikes against South Korean border units unless the broadcasts are halted immediately.