The US and South Korea are conducting annual military drills which consistently infuriate Pyongyang, despite appeals to halt the exercise.
US and South Korean troops have started their annual drills that come after President Donald Trump and North Korea exchanged warlike rhetoric in the wake of the North's two intercontinental ballistic missile tests last month.
The Ulchi Freedom Guardian drills are largely computer-simulated war games held every summer and have drawn furious responses from North Korea, which views them as an invasion rehearsal. Pyongyang's state media on Sunday called this year's drills a "reckless" move that could trigger the "uncontrollable phase of a nuclear war."
Despite the threat, US and South Korean militaries launched this year's 11-day training on Monday morning as scheduled. The exercise involves 17,500 American troops and 50,000 South Korean soldiers, according to the US military command in South Korea and Seoul's Defence Ministry.
The exercises, the world's largest computerised war-simulated drills, include representatives from seven member countries of the United Nations Command - Australia, Canada, United Kingdom, New Zealand, Netherlands, Denmark and Colombia - allied nations in the Korean War between 1950 and 1953.
No field training like live-fire exercises or tank manoeuvring is involved in the Ulchi drills, in which alliance officers sit at computers to practice how they engage in battles and hone their decision-making capabilities. The allies have said the drills are defensive in nature.
South Korea's President Moon Jae-in said on Monday that North Korea must not use the drills as a pretext to launch fresh provocation, saying the training is held regularly because of repeated provocations by Pyongyang.
North Korea typically responds to South Korea-US military exercises with weapons tests and a string of belligerent rhetoric. During last year's Ulchi drills, North Korea test-fired a submarine-launched ballistic missile that flew about 500km in the longest flight by that type of weapon. Days after the drills, the North carried out its fifth and biggest nuclear test to date.
Last month North Korea test-launched two ICBMs at highly lofted angles, and outside experts say those missiles can reach some US parts like Alaska, Los Angeles or Chicago if fired at normal, flattened trajectories.
Earlier this month, President Trump pledged to answer North Korean aggression with "fire and fury." North Korea, for its part, threatened to launch missiles toward the American territory of Guam before its leader Kim Jong Un backed off.