Thermal images of North Korea's main nuclear plant suggest that Pyongyang has reprocessed more weapons-grade plutonium than previously thought.

The information, which comes from 38 North, a Washington-based think tank connected to Johns Hopkins University, likely means that the North can expand its nuclear weapons stockpile more rapidly than has been estimated up to now.

"The Radiochemical Laboratory operated intermittently and there have apparently been at least two unreported reprocessing campaigns to produce an undetermined amount of plutonium that can further increase North Korea's nuclear weapons stockpile," it said.

These conclusions have been drawn on the basis of satellite images showing the radiochemical laboratory at the Yongbyon nuclear facility between September and the end of June.

Worrying security threat

The monitors said images of the uranium enrichment facility at Yongbyon may also indicate an increase in the North's supply of enriched uranium, its other source of bomb-making fuel.

There were also signs of short-term activity at North Korea's Experimental Light Water Reactor, which is also cause for concern, according to 38 North.

Images from the radiochemical laboratory indicate at least two processing cycles that were previously unknown. The aim is to produce "an undetermined amount of plutonium that can further increase North Korea's nuclear weapons stockpile," according to a statement from the monitors.

It's a worrying development for US officials, who see North Korea as one of the world's top security threats.

Yongbyon: silenced in 2007

Pyongang deactivated the Yongbyon reactor in 2007 as part of an aid-for-disarmament agreement, but began renovating the plant after the North's third nuclear test in 2013.

Increased thermal activity was also noted at Yongbyon's uranium enrichment facility, but it was unclear whether this indicated an effort to increase supplies, possibly for weapons, or if it was part of maintenance operations.

The researchers also reported that analysis of thermal patterns from a probable isotope/tritium production facility at the site suggested that the plant was likely not producing tritium.

Tritium is a key component used for making sophisticated thermonuclear weapons with substantially greater yields than those made only of plutonium and uranium.

New US sanctions?

Frustrated that China, North Korea's main trading partner, has done little to rein in Pyongyang, the US administration under President Donald Trump could impose new sanctions on small Chinese banks and other companies doing business with the North in the coming weeks, according to two senior US officials who spoke on condition of anonymity.

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Meanwhile, the US ambassador to the United Nations, Nikki Haley, has been seeking to overcome resistance from China and Russia to a UN Security Council resolution imposing stiffer international sanctions on Pyongyang.

Experts at 38 North estimated in April that North Korea could have as many as 20 nuclear bombs and could produce one more each month.

bik/tj (Reuters, AFP)