North Korea has offered its first glimpse into a secretive facility used to produce weapons-grade uranium, as state media reported on Friday that leader Kim Jong Un visited the area and called for stronger efforts to "exponentially" increase the number of his nuclear weapons.

It's unclear if the site is at the North's main Yongbyon nuclear complex, but it marks the first public images of the facility and the first disclosure of one since North Korea showed one to visiting American scholars in 2010.

In a response, South Korea's Ministry of Unification said it strongly condemned the unveiling and Mr Kim's vows to boost his country's nuclear capability.

A ministry statement said North Korea's "illegal" pursuit of nuclear weapons was in "clear violation of a number of UN Security Council resolutions".

It said North Korea must realise it cannot win anything with its nuclear program.

Facility control room, centrifuges broadcast

During a visit to the Nuclear Weapons Institute and the production base of weapons-grade nuclear materials, Mr Kim expressed "great satisfaction repeatedly over the wonderful technical force of the nuclear power field" held by North Korea, the state-controlled Korean Central News Agency reported.

KCNA said that Mr Kim visited the control room of the uranium enrichment base and a construction site that would expand its capacity for producing nuclear weapons.

North Korean state media photos showed Mr Kim being briefed by scientists while walking along long lines of metal centrifuges — the machines which enrich the uranium.

KCNA did not say when Mr Kim visited the facility or where it is located.

But International Atomic Energy Agency chief Rafael Grossi said on Monday that the UN nuclear watchdog had observed activity consistent with the operation of a reactor and the reported centrifuge enrichment facility at Yongbyon.

KCNA said Mr Kim stressed the need to further augment the number of centrifuges to "exponentially increase the nuclear weapons for self-defence", a goal he has repeatedly stated in recent years.

It said Mr Kim ordered officials to push forward with the introduction of a new-type centrifuge, which has reached its completion stage.

Mr Kim said North Korea needs greater defence and pre-emptive attack capabilities because "anti-[North Korea] nuclear threats perpetrated by the US imperialists-led vassal forces have become more undisguised and crossed the red-line".

New footage 'valuable source' for outside analysts

Ankit Panda, Stanton Senior Fellow in the Nuclear Policy Program at the Carnegie Endowment for International Peace, said the unveiling could provide outsiders with the information needed to estimate the amount of nuclear ingredients North Korea has produced.

"For analysts outside the country, the released images will provide a valuable source of information for rectifying our assumptions about how much material North Korea may have amassed to date," he said.

"Overall, we should not assume that North Korea will be as constrained as it once was by fissile material limitations.

"This is especially true for highly enriched uranium, where North Korea is significantly less constrained in its ability to scale up than it is with plutonium."

He added that Friday's report indicated North Korean tactical nuclear weapons designs "may primarily rely on uranium for their cores", and that it could more easily stockpile uranium than plutonium.

The North Korean photos released on Friday showed about 1,000 centrifuges.

When operated year-round, they would be able to produce around 20 to 25 kilograms of highly enriched uranium — enough to create a single bomb, according Asan Institute for Policy Studies Research Fellow Yang Uk.

The centrifuges seen in the photos appear smaller and shorter than the types North Korea has seemingly previously used, Korea Institute for Defense Analysis Research Fellow Lee Sang-kyu said.

This suggested North Korea had developed its own centrifuges to enhance separation capabilities.

The photos also confirmed that the North is using a cascade system where large numbers of centrifuges are interconnected to achieve highly enriched uranium, he added.

The centrifuges are likely made from carbon fibre and could allow North Korea to produce five to 10 times more highly enriched uranium than existing ones, Science & Technology Policy Institute Research Fellow Lee Choon Geun said.

Weapons testing could be used to influence US election

But the latest unveiling is more likely an attempt to apply more pressure on the US and its allies than it is a boon for analysts.

Since 2022, North Korea has sharply ramped up weapons testing activities to expand and modernise its arsenal of nuclear missiles targeting the US and South Korea.

Analysts say North Korea could perform nuclear test explosions or long-range missile tests ahead of the US presidential election in November with the intent to influence the outcome and increase its leverage in future dealings with the US.

"Overall, the message they are trying to send is that their nuclear capability is not just an empty threat, but that they are continuing to produce [bomb fuel]," Mr Yang said.

North Korea conducted test-launches of multiple short-range ballistic missiles on Thursday.

In an apparent reference to those launches, KCNA said Mr Kim had supervised test-firing of nuclear-capable 600mm multiple rockets to examine the performance of their new launch vehicles.

North Korea's nuclear arsenal unknown

North Korea first showed a uranium enrichment site in Yongbyon to the outside world in November 2010, when it allowed a visiting delegation of Stanford University scholars led by nuclear physicist Siegfried Hecker to tour its centrifuges.

North Korean officials then reportedly told Mr Hecker that 2,000 centrifuges were already installed and running at Yongbyon.

However, Friday's report marked the first and only photographs of the equipment.

North Korea has previously shown photos of what it says were nuclear warheads. It has conducted six underground nuclear tests between 2006 and 2017.

Satellite images in recent years have indicated North Korea was expanding a uranium enrichment plant at its Yongbyon nuclear complex.

Nuclear weapons can be built using either highly enriched uranium or plutonium, and North Korea has facilities to produce both at Yongbyon.

It's not clear exactly how much weapons-grade plutonium or highly enriched uranium has been produced at Yongbyon and where North Korea stores it.

Uranium is a radioactive element that exists naturally. To make nuclear fuel, raw uranium undergoes processes that result in a material with an increased concentration of the isotope uranium-235.

In 2018, Mr Hecker and Stanford University scholars estimated North Korea's highly enriched uranium inventory was 250 to 500 kilograms — sufficient for 25 to 30 nuclear devices.

Estimates of the number of North Korean nuclear weapons varies widely. 

A July report by the Federation of American Scientists concluded that the country may have produced enough fissile material to build up to 90 nuclear warheads, but that it has likely assembled closer to 50.

Some US and South Korean experts speculate North Korea is covertly running at least one other uranium-enrichment plant.

In 2018, a top South Korean official told parliament that North Korea was estimated to have already manufactured up to 60 nuclear weapons.

Estimates on how many nuclear bombs North Korea can add every year range from six to 18.

AP/Reuters