While the exercises will mainly revolve around computer-simulated war games, North Korea has described them as highly provocative, claiming that they are a rehearsal for invasion.
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China's Foreign Ministry said that military exercises between South Korean and US forces were not beneficial to the easing of tensions on the Korean peninsula.
The United States and South Korea Monday kicked off their annual Ulchi-Freedom Guardian joint military drill amid high tension on the Korean Peninsula and repeated appeals for restraint on both sides by many countries.
In turn, Mr Kim has threatened to fire missiles towards the United States territory of Guam in the Pacific Ocean.
Called Ulchi Freedom Guardian (UFG), the joint drills have their roots in a 1968 raid on South Korea's Blue House presidential complex, when Unit 124 of the North Korean army secretly entered South Korea and unsuccessfully attempted to assassinate the then president, Park Chung-hee.
"In the case of North Korea, we can't see everything in real time but the military deduces the locations of North Korean troops, including the leadership during the exercise".
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Earlier this month, US President Donald Trump vowed to respond to North Korea's actions with "fire and fury".
Leader Kim Jong-Un last week delayed the Guam strike plan, but warned it could go ahead depending on Washington's next move. The exercise was initiated in 1976 and is conducted annually during August or September.
According to the south Korean agency Yonhap, the two allies are studying the opportunity to abandon their initial project, to deploy two aircraft carriers near the peninsula in the framework of this exercise.
But the United States defence secretary, James Mattis, said the smaller troop numbers were "by design to achieve the exercise objectives" on Sunday and denied Washington had cut troop numbers in an attempt to ease tensions with Pyongyang.
"This right now is an exercise to make certain that we're ready to defend South Korea and our allies over there", he added.
On the eve of the UFG drills, North Korea said the USA was "pouring gasoline on fire".
The North Korean message in Rodong Sinmun used ominous language, "The Trump group's declaration of the reckless nuclear war exercises against the DPRK.is a reckless behavior driving the situation into the uncontrollable phase of a nuclear war", the editorial stated using the acronym for Democratic People's Republic of Korea, North Korea's official name.