The North has long been known to hold a "significant" amount of highly enriched uranium, the key material used to produce nuclear warheads, according to South Korea's defence ministry.

But in a rare public confirmation, South Korea's unification minister said that "intelligence agencies estimate Pyongyang's stockpile of highly enriched uranium - more than 90 percent pure - at up to 2,000 kilogrammes."

"Even at this very hour, North Korea's uranium centrifuges are operating at four sites," Chung Dong-young told reporters.

"Only five to six kg of plutonium is enough to build a single nuclear bomb," said Chung, adding that 2,000 kg of highly enriched uranium, which could be reserved solely for plutonium production, would be "enough to make an enormous number of nuclear weapons".

Chung said that "stopping North Korea's nuclear development is an urgent matter", but argued that sanctions will not be effective and that the only solution lies in a summit between Pyongyang and Washington.

North Korean leader Kim Jong Un said this week that he was open to US talks provided he can keep his nuclear arsenal.

North Korea, which conducted its first nuclear test in 2006 and is under rafts of UN sanctions for its banned weapons programmes, has never publicly disclosed details of its uranium enrichment facility until last September.

The country is believed to operate multiple uranium enrichment facilities, Seoul's spy agency has said, including one at its Yongbyon nuclear site, which Pyongyang purportedly decommissioned after talks -- although it later reactivated the facility in 2021.