New images of North Korea's main nuclear facility show that the isolated regime has apparently produced more plutonium for its weapons programme than previously thought, a US monitor said, as tensions soar over Pyongyang's ambitions.

The respected 38 North website, a monitoring project linked to Johns Hopkins university, said Friday that thermal imagery of the Yongbyon nuclear complex appeared to show that Pyongyang had reprocessed spent fuel rods at least twice between last September and June this year.

"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.

North Korea deactivated the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after Pyongyang'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 a push to increase supplies, possibly for weapons, or if it was part of maintenance operations, the website said.

The researchers also said 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 far greater yields than those made only of plutonium and uranium.

North Korea has conducted five underground nuclear tests since 2006, and carried out its first successful test of an intercontinental ballistic missile last week. 

North Korea's foreign ministry spokesman said Friday "the resounding success" of the test demonstrated Pyongyang's ability to "annihilate the US by a single blow to the very heart of its mainland in case it fails to act with discretion".

The United States is inviting its "ultimate doom" by pushing for ramped up UN sanctions against the North, the spokesman was quoted as saying by Pyongyang's state media.

North Korea, which says it needs nuclear weapons to defend itself against the threat of invasion, is subject to multiple sets of United Nations sanctions over its weapons programmes.

The latest launch triggered a new round of condemnation and anger, with the US and its allies seeking toughened measures at the Security Council.

Source: AFP