North Korea warned the United States on Tuesday that it could face "merciless revenge" after ignoring warnings not to go ahead with an annual joint military exercise with South Korea, according to Pyongyang's official media.

"The U.S. will be wholly held accountable for the catastrophic consequences to be entailed by such reckless aggressive war maneuvers, as it chose a military confrontation with the DPRK," a spokesman for the Panmunjeom mission of the Korean People's Army said in a statement carried by the Korean Central News Agency.

DPRK is the acronym of North Korea's formal name, the Democratic People's Republic of Korea.

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The statement was in response to the 10-day exercise, codenamed Ulchi-Freedom Guardian, involving U.S. and South Korean troops that began on Monday. The drill is being conducted under heightened tensions on the Korean Peninsula following North Korea's two intercontinental ballistic missile tests last month.

KCNA reported that "what should not be overlooked" is that the exercise reflects an operation, dubbed "OPLAN 5015," which it described as "an aggressive war scenario to carry out a preemptive strike against the DPRK."

While the United States and South Korea claim the exercise is entirely defensive, Pyongyang says it is a rehearsal for an invasion of the North.

Ahead of the drill, U.S. brass visited South Korea including Adm. Harry Harris, the top U.S. military commander in the Asia-Pacific region. Harris arrived in Seoul on Sunday to observe the exercise.

The statement also took issue with such visits which are "increasing the gravity of the situation."

KCNA said the United States should not forget that its moves are being closely watched by North Korea.