South Korea said on Monday its push to obtain nuclear-powered submarines was not intended to fuel a broader arms race, and that it had reassured Washington and Beijing that they were needed to respond to North Korea's submarine program.

President Donald Trump said on Thursday he has given South Korea approval to build a nuclear-powered submarine, a dramatic move that would admit Seoul to a small club of nations possessing such vessels.

"It is not to generate more of an arms race ... but the result of persuading China and the U.S. that North Korea has announced (its own) nuclear submarines and we need to prepare correspondingly," presidential chief of staff Kang Hoon-sik told reporters.

South Korea's national security adviser Wi Sung-lac had said on Saturday that Seoul had asked for U.S. help so it could use fuel for its proposed nuclear-powered submarines, since it would be for military purposes, and the U.S. had approved it.

Asked whether that meant the U.S. would supply fuel for nuclear-powered submarines or if South Korea could enrich uranium for military purposes, the spokesperson for South Korea's defense ministry said on Monday more discussion was needed between Washington and Seoul on the issue.