State media in the North said: The joint exercise is the most explicit expression of hostility against us, and no one can guarantee that the exercise won't evolve into actual fighting.
Last week North Korea appeared to back down from a threat to send missiles towards the US Pacific island of Guam, but said it would watch US actions.
Moon, who took office in May, announced on his 100th day in office that US and South Korean policies are aligned on North Korea.
South Korea set for divisive military drills
South Korean President Moon Jae-in again urged the United States Monday to refrain from taking any action against North Korea that could start an armed conflict on the peninsula, saying even the slightest conflict here may cost the lives of many South Koreans and USA troops.
There will be no field training during the current exercise, according to U.S. Forces Korea. North Korean leader Kim Jong Un has also threatened the United States with a nuclear attack.
That sparked an intense warning by President Donald Trump that Washington could rain "fire and fury" on the North.More news: 'Morata good enough to replace Costa - but Chelsea need more signings!'
The exercises this week, known as Ulchi-Freedom Guardian, follow a North Korean threat this month to launch four ballistic missiles into waters near Guam, home to major USA military bases in the Western Pacific.
North Korea's rapid progress in developing nuclear weapons and missiles capable of reaching the US mainland has fueled a surge in tension.
It described North Korea as the "strongest possessor" of intercontinental ballistic missiles capable of striking the U.S. mainland from anywhere. "But he said even the slightest military action will lead to a clash between the South and North, and it will threaten many lives of not only South Koreans but foreigners, including USA forces stationed in South Korea", Park Soo-hyun, a spokesman for Seoul's presidential office Cheong Wa Dae, said at a press briefing. "North Korea should not use this as a pretext for provocation". China is a key North Korean ally. "Even a very limited military option would eventually lead to an armed clash between South and North Korea", Moon's office quoted him as telling the delegation, led by Sen. The exercises include computer simulations carried out in a large bunker south of Seoul meant to check the allies' readiness to repel aggressions by the North. Russian Federation has also asked for the drills to stop but the United States has not backed down.More news: Thousands in United States witness total solar eclipse
About 17,500 USA service members are participating in the exercise this month, down from 25,000 past year, according to the Pentagon.
Choe Sang-Hun and Austin Ramzy are New York Times writers.More news: Paul Ryan: Trump 'messed up' on Charlottesville response
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