Kim Jong Un's sister warns North Korea ready to act against U.S., South Korea

Kim Yo Jong, the influential sister of North Korean leader Kim Jong Un, warned Tuesday that her nation is prepared for "quick, overwhelming action" against South Korea and the United States.

The U.S. flew a nuclear-capable B-52 bomber during a training exercise with South Korea over the Korean Peninsula on Monday, sparking Kim's response, the Associated Press reported.

"We keep our eye on the restless military moves by the U.S. 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," she said, according to state media.

"The demonstrative military moves and all sorts of rhetoric by the U.S. and South Korea, which go so extremely frantic as not to be overlooked, undoubtedly provide (North Korea) with conditions for being forced to do something to cope with them," Kim also said. 

Hours after her statement, North Korea's army said its front-line artillery units were placed on alert and surveillance activities were heightened after detecting a live-fire artillery drill in a South Korean border town Tuesday morning.