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.

South Korea condemned North Korea's unveiling of its uranium enrichment facility and will never accept Pyongyang's possession of nuclear weapons, the South's unification ministry said.

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.