The country, 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.

Such facilities produce highly enriched uranium -- which is needed to produce nuclear warheads -- by spinning the original material in centrifuges at high speeds.

Kim toured the Nuclear Weapons Institute and the "production base of weapon-grade nuclear materials," the official Korean Central News Agency reported, without giving the location of the facility or the date of the visit.

Kim "stressed the need to further augment the number of centrifuges in order to exponentially increase the nuclear weapons for self-defence," state media reported, publishing images of Kim inspecting rows of centrifuges.

Kim "acquainted himself with the production of nuclear warheads and current nuclear materials," the report said.

The North Korean leader was briefed about the facility "dynamically producing nuclear materials by studying, developing and introducing all the system elements including centrifugal separators," KCNA said.

Kim urged the facility to "push forward the introduction of a new-type centrifuge... so as to further strengthen the foundation for producing weapon-grade nuclear materials".

Kim also "stressed the need to set a higher long-term goal in producing nuclear materials necessary", added KCNA.

North Korea's nuclear weapons programmes are banned by UN sanctions, but the country has long flouted the restrictions, thanks in part to support from allies Russia and China.

- Message for US? -

Experts said the sudden public disclosure of the North's uranium enrichment facility could be intended to impact the US presidential election in November.

The images are "a message to the next administration that it will be impossible to denuclearise North Korea", Hong Min, a senior analyst at the Korea Institute for National Unification, told AFP.

"It is also a message demanding other countries to acknowledge North Korea as a nuclear state," he added.

It is unlikely that the disclosure will be quickly followed by another nuclear test, he said.

Pyongyang last month said a record downpour in late July had killed an unspecified number of people, flooded dwellings and submerged swathes of farmland in its northern regions near China.

38 North, a North Korean analysis programme run by the Stimson Centre think-tank, reported on Wednesday that North Korea's main nuclear test site had been damaged by floodwaters.

North Korea's main nuclear test site "is in very bad condition. All roads and railways have been lost due to rain damage, and the ground is very weakened," Hong added.

Relations between North and South Korea are at one of their lowest points in years, with the North recently announcing the deployment of 250 ballistic missile launchers to its southern border.

The North has also been bombarding the South with trash-carrying balloons, including a five-day straight blitz last week.

On Thursday, Seoul said the North had fired multiple short range ballistic missiles into waters east of the Korean peninsula.

But KCNA said in a separate dispatch Friday that this had been a test of a "new-type 600mm multiple rocket launcher" which was overseen by Kim.