The powerful sister of North Korean leader Kim Jong-un slammed South Korea and Ukraine on Tuesday for what she claimed was a military provocation against Pyongyang, likening the two countries to "bad dogs bred by the US."

The statement by Kim Yo-jong, vice department director of the central committee of the ruling Workers' Party, came days after South Korea's spy agency said the North was sending troops to fight alongside Russia in the war in Ukraine.

"A military provocation against a nuclear weapons state may be led to horrible situation, unimaginable for politicians and military experts in any big or small country in the world with their normal thinking to experience," she said in the statement carried by the Korean Central News Agency.

"So, there will be no such instance except one recently made by lunatics of the ROK and Ukraine. Such deed can be done only by lunatics in the Seoul and Kiev regimes," she said, referring to South Korea by the acronym of its formal name, Republic of Korea.

Kim said Seoul and Kyiv are "exact counterparts" in making "reckless remarks" about nuclear weapons states, adding that "it seems to be a common feature of bad dogs bred by the US."

She also noted that an investigation is under way by North Korean detective agencies to determine additional details about the drones Pyongyang has accused Seoul of sending over the capital carrying anti-North Korea propaganda leaflets.

"No one knows how our retaliation and revenge will be completed," she said, claiming "a lot of political motivational rubbish sent by" South Korea was discovered in "many parts" of the North the previous day as well. (Yonhap)