Thermal images of North Korea's main nuclear facility reveal that Kim Jong-un may have produced more plutonium than previously thought in a bid to expand his stockpile of atomic weapons, according to US researchers.

The rogue state has carried out at least two secret campaigns to reprocess radioactive material at the Yongbyon nuclear plant in Nyongbyon County, it is claimed.

Satellite images of one of the plant's installations revealed an increase in thermal activity that could be the result of centrifuge operations aimed at enriching uranium so it can be used in bombs, it is said.

A satellite image of the Yongbyon nuclear plant in Nyongbyon County, 56 miles north of Pyongyang (Airbus Defense &amp; Space and 38 North. Includes material Pleiades © CNES 2017 Distribution Airbus DS / Spot Image, all rights reserved) (Airbus Defense & Space and 38 North. Includes material Pleiades © CNES 2017 Distribution Airbus DS / Spot Image, all rights reserved)

The analysis will raise fears that Kim intends to manufacture more nuclear weapons, days after North Korean officials described a US training exercise near their border as a "dangerous military gambit of warmongers who are trying to ignite the fuse of a nuclear war on the peninsula".

Researchers at North Korea monitoring project at 38 North said there was "cause for concern" because some of the thermal images showed that a reactor may have been operational at the Yongbyon nuclear plant between December 2016 and January this year.

The Washington-based think-tank scrutinised images captured by United States Geological Survey satellite Landsat 7 and produced a report titled Probable Production of Additional Plutonium for Nuclear Weapons.

It states that there have been two or more unreported reprocessing campaigns at the Yongbyon plant's radiochemical installation "to produce an undetermined amount of plutonium that can further increase North Korea’s nuclear weapons stockpile".

A thermal satellite image of the Yongbyon nuclear plant in Nyongbyon County, 56 miles north of Pyongyang. (Airbus Defense &amp; Space and 38 North. Includes material Pleiades © CNES 2017 Distribution Airbus DS / Spot Image, all rights reserved) (Airbus Defense & Space and 38 North. Includes material Pleiades © CNES 2017 Distribution Airbus DS / Spot Image, all rights reserved)

It said thermal activity at the uranium enrichment facility could be a result of centrifuge operations that "would increase the North’s enriched uranium inventory", and give the regime more fuel for bombs.

But researchers concluded that the elevated patterns of thermal activity could also be a result of maintenance operations such as the heating of pipes to prevent them freezing.

Thermal readings at what is thought to be the site's isotope and tritium production facility remained consistent over several months, suggesting that it was "likely not producing tritium, which is an essential isotope used in the production of boosted yield nuclear weapons and hydrogen bombs".

"Based on imagery alone, it is not possible to conclude whether the plant is producing low or highly enriched uranium," the report added.

"Regardless, any activity at the Experimental Light Water Reactor is cause for concern and bears continued monitoring."

North Korea has justified weapons tests as "legitimate and justified measures" amid increasing "threats of nuclear war" against Pyongyang by Washington.

In pictures: North Korea military drill Show all 8 1 /8 In pictures: North Korea military drill In pictures: North Korea military drill North Korea's leader Kim Jong Un watches a military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video

The dictatorship is banned from testing or developing missiles under UN resolutions, but has been working to develop a nuclear-tipped missile capable of hitting the US, ignoring repeated warnings from the international community.