South Korea and the United States are discussing possible joint exercises using U.S. nuclear assets, South Korean President Yoon Suk-yeol said, as North Korean leader Kim Jong Un labelled the South its "undoubted enemy" in flaring cross-border tensions.

Yoon's comments, in a newspaper interview published on Monday, come after he called for "war preparation" with an "overwhelming" capability, following a year marked by the North's record number of missile tests, and the intrusion of North Korean drones into the South last week.

"The nuclear weapons belong to the United States, but planning, information sharing, exercises and training should be jointly conducted by South Korea and the United States," Yoon said in the interview with the Chosun Ilbo newspaper.