North Korea for the first time showed images of the centrifuges that produce fuel for its nuclear bombs on Friday, as leader Kim Jong-un visited a uranium enrichment facility and called for more weapons-grade material to boost the arsenal.

The state media report on Kim's visit to the Nuclear Weapons Institute and a production base for weapon-grade nuclear materials was accompanied by the first photos of the centrifuges, providing a rare look inside North Korea's nuclear programme, which is banned under multiple United Nations Security Council resolutions.

The photos showed Kim walking between long rows of metal centrifuges, the machines that enrich uranium. The report did not make clear when the visit occurred nor the facility's location.

Kim urged workers to produce more materials for tactical nuclear weapons, saying the country's nuclear arsenal is vital for confronting threats from the United States and its allies.

The weapons are needed for "self-defence and the capability for a preemptive attack," he said.

The North Korean leader said "anti-DPRK nuclear threats" from the "U.S. imperialists-led vassal forces" have crossed the red-line, according to the report.

North Korea is believed to have several sites for enriching uranium. Analysts say commercial satellite imagery has shown construction in recent years at the main Yongbyon Nuclear Scientific Research Center, including its uranium enrichment plant, suggesting possible expansion.

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.

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

Kim stressed the need to increase the number of centrifuges so as to "exponentially increase" the nuclear weapons and expand use of a new type of centrifuge to further strengthen the production of weapon-grade nuclear materials.

The new type of centrifuge shows North Korea is advancing its fuel cycle capabilities, said Ankit Panda of the U.S.-based Carnegie Endowment for International Peace.

"Kim also appears to suggest that North Korean tactical nuclear weapons designs may primarily rely on uranium for their cores," he said.

This is notable because North Korea is more able to scale up its highly enriched uranium stockpiles, Panda said, compared to the more complicated process for plutonium.

North Korea invited some foreign scientists to view a centrifuge facility at Yongbyon in 2010, but Jenny Town of the U.S.-based Stimson Center said Friday's report is the first and only photographs of the equipment.

"It shows how advanced their enrichment capability has become, which gives greater credibility to both their ability and commitment to increasing their nuclear weapons arsenals," she said.

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

Estimates of the number of North Korean nuclear weapons varies widely. In July a 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.

Kim also oversaw the test launch of a new 600mm multiple launch rocket system on Thursday and visited an army training base on Wednesday, according to separate KCNA reports.

In a statement carried by KCNA, North Korea's foreign ministry institute spokesperson criticized a recent defence ministerial meeting between member states of the U.S.-led United Nations Command in Seoul, calling it a "war organization."

Germany joined the command last month, becoming the 18th nation in a group that helps police the heavily fortified border with North Korea and has committed to defend the South in the event of a war.