About 17,500 US service members will participate, with about 3,000 coming from installations outside South Korea, the officials 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", said an editorial carried by the North's official Rodong Sinmun newspaper.
South Korean army soldiers hold their weapons during an anti-terror exercise carried out as part of Ulchi Freedom Guardian exercise in 2010 What is the USA and South Korea Ulchi-Freedom Guardian 10-day military drill?
The Ulchi Freedom Guardian exercise will begin on Monday and will conclude on August 31.
"If the United States is lost in a fantasy that war on the peninsula is at somebody else's door far away from them across the Pacific, it is far more mistaken than ever", the report said.
Pyongyang also declared that its Army can target the U.S. any time and neither Guam, Hawaii nor the United States mainland can "dodge the merciless strike".
But it was later reported Kim had put his plans to launch four Hwasong missiles towards Guam on ice so he could see what Donald Trump's next move would be.
Admiral Harry Harris the commander of the U.S. Pacific Command is now visiting South Korea to inspect the joint exercise.
Tensions between North Korea and the US have risen in recent weeks as their respective leaders' exchanged harsh rhetoric.
Pyongyang then threatened to fire a salvo of missiles towards the U.S. territory of Guam - a plan that leader Kim Jong-un last week delayed, but warned could go ahead depending on Washington's next move.
The new top military officer issued a stern warning that North Korea will face merciless retaliation for any attack. The North later walked back that threat, saying that it would instead keep an eye on the situation.
"If the enemy provokes, [our military] will retaliate resolutely and strongly to make it regret bitterly", said General Jeong Kyeong-doo, chairman of the Joint Chiefs of Staff, in his inauguration speech.
Mattis also made clear the US' willingness to use force if North Korea steps out of line.
During the hearing, Jeong also rejected the conservative parties' demand to consider the redeployment of US tactical nukes that were withdrawn when the two Koreas signed a joint declaration for a nuclear-free peninsula in 1991.