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 "according to estimates by experts including the Federation of American Scientists (FAS) they (North Korea) currently hold around 2,000 kilogrammes of highly enriched uranium at a purity of 90 percent or higher."

"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".

Enriched uranium is the key ingredient for making nuclear bombs, as it can be turned into plutonium through combustion in a nuclear reactor.

Enrichment must be pushed to more than 90 percent, the concentration termed weapons-grade, that is needed for the critical mass to set off the chain reaction leading to a nuclear explosion.

According to the International Atomic Energy Agency (IAEA), 42 kilogrammes of highly enriched uranium is needed for one nuclear weapon; 2,000 kilogrammes would be enough for roughly 47 nuclear bombs.

"Urgent matter"

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, according to a report by the official Korean Central News Agency.

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.

The minister also blamed the previous administration, saying that by designating the North as the "main enemy" and insisting on denuclearisation first, it had effectively allowed North Korea's nuclear capabilities "to expand without limit."

South Korea's President Lee Jae Myung, who took office in June, has vowed a more dovish approach towards Pyongyang compared with his hawkish predecessor Yoon Suk Yeol.

Lee vowed Tuesday at the United Nations to work to end the "vicious cycle" of tensions with the North as he promised not to seek regime change.