North Korea on Tuesday denounced an annual joint military exercise that South Korea and the United States launched this week, warning they will pay a "dear price" for what it called their large-scale war drills.

An unnamed spokesperson at the North's defense ministry issued a statement condemning the annual Freedom Shield exercise that began Monday for an 11-day run to strengthen deterrence against North Korea's nuclear and missile threats.

"The large-scale war drills staged by the world's biggest nuclear weapons state and more than 10 satellite states against a state in the Korean peninsula where a nuclear war may be ignited even with a spark, can never be called 'defensive,'" read the English-language statement carried by the Korean Central News Agency.

The official said North Korea's army will monitor the enemies' "adventurist acts" and conduct "responsible military activities" to bring the unstable security situation under control.

"The US and the Republic of Korea will be made to pay a dear price for their false choice while realizing that it causes their security uneasiness at a serious level every moment," the spokesperson said.

This year's military drills come as North Korea has ramped up weapons tests in recent months with the launches of cruise missiles from land and sea and artillery firing near the tense western sea border.

On Monday, South Korean and US reconnaissance aircraft made a sortie in their apparent mission to closely monitor North Korea. During the military exercise, the US is likely to send its strategic military assets to the Korean Peninsula in a show of force.

Pyongyang has long denounced the allies' joint military drills as rehearsals for an invasion and used them as a pretext for provocations. Seoul and Washington have said their military exercises are defensive in nature. (Yonhap)