The United States and South Korea began large-scale joint military exercises on March 7.
More than 300,000 South Korean and 15,000 American troops are to take part in the annual drills, which run until the end of April.
South Korea said the exercises would be the largest U.S.-South Korean war games following Pyongyang's nuclear test in January and rocket launch last month that triggered new UN sanctions.
Pyongyang threatened to carry out a "preemptive nuclear strike" against the two allies if they push ahead with the drills.
Such rhetoric is not uncommon, and military analysts doubt North Korea has the ability to put nuclear warheads on its missiles.
Responding to the North's threat, South Korea warned Pyongyang against any "rash act that brings destruction upon itself."
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