South Korea, U.S. discussing nuclear exercises as tensions flare with North -Yoon

SEOUL, Jan 2 (Reuters) - Seoul and Washington are discussing possible joint exercises using U.S. nuclear assets, South Korean President Yoon Suk-yeol said, while North Korean leader Kim Jong Un branded the South its "undoubted enemy" in flaring cross-border tensions.

Yoon's comments, in a newspaper interview published on Monday, followed his call for "war preparation" with an "overwhelming" capability, after a year of a record number of North Korean 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.

The newspaper quoted Yoon as saying the joint planning and exercises would be aimed at a more effective implementation of the U.S. "extended deterrence" and that Washington was also "quite positive" about the idea.

The term "extended deterrence" means the ability of the U.S. military, particularly its nuclear forces, to deter attacks on U.S. allies.

The White House and State Department did not respond to a request for comment on Yoon's remarks. A Pentagon spokesperson said: "We have nothing to announce today."

The United States has long had an extended deterrence dialogue with Japan to talk about nuclear issues and initiated the same dialogue with South Korea in 2016, said Thomas Countryman, the former acting undersecretary of state for arms control who chaired the dialogue's first meeting.

"It's not immediately clear what in President Yoon's statement is new and what is a rephrasing of things that are already happening," Countryman said on Monday in a phone interview.

Now board chairman of the Arms Control Association, Countryman said Yoon's comments, directed at the South Korean people, appeared to be in response to what Countryman called North Korea's provocations and rhetoric.

"I do see this as an effort by both President Yoon and the Biden administration to reassure the government and the people of South Korea, that the U.S. commitment remains solid."

Yoon's remarks were published a day after North Korean state media reported that its leader Kim had called for developing new intercontinental ballistic missiles (ICBMs) and an "exponential increase" of the country's nuclear arsenal.

At a meeting of the ruling Workers' Party last week, Kim said South Korea had become the North's "undoubted enemy" and rolled out new military goals, hinting at another year of intensive weapons tests and tension.

Inter-Korean ties have long been testy but have been even more frayed since Yoon took office in May, promising a tougher stance on the North.

On Sunday, North Korea fired a short-range ballistic missile off its east coast, in a rare late-night, New Year's Day weapons test, following three ballistic missiles launched on Saturday.