Kim Yo-jong, the sister of North Korean leader Kim Jong-un, warned Tuesday the North is ready to take "overwhelming" actions against military activities by the United States and South Korea, as its rivals are staging joint military drills involving American strategic assets.

Kim also warned the North will regard any US attempt to intercept a missile that it will fire as a "clear declaration of war" against the reclusive nation, according to the North's Korean Central News Agency.

"As already clarified, we keep our eye on the restless military moves by the US forces and the south Korean puppet military and are always on standby to take appropriate, quick and overwhelming action at any time according to our judgment," Kim said in an English-language statement carried by the state-run media.

Seoul and Washington conducted combined air drills Monday, involving at least one US nuclear-capable B-52H strategic bomber, in the wake of Friday's similar drills mobilizing a US B-1B bomber.

The allies also plan to stage the Freedom Shield exercise, a computer-simulated command post training, from March 13-23 in a bid to bolster deterrence against the North's nuclear and missile threats.

Kim also took issue with a news report about the commander of the US Indo-Pacific Command. A local newspaper reported Monday that Adm. John Aquilino warned if the North fires an intercontinental ballistic missile toward the Pacific, the US will immediately intercept it.

"It will be regarded as a clear declaration of war against the DPRK, in case such military response as interception takes place against our tests of strategic weapons that are conducted without being detrimental to the security of neighboring countries in the open waters and air which do not belong to the US jurisdiction," she said.

DPRK stands for the North's official name, the Democratic People's Republic of Korea.

Kim has recently threatened that the North could use the Pacific as its "firing range" in response to joint military drills by the US and the South. (Yonhap)