North Korea might have continued to engage in uranium enrichment activities in spite of its pledge to give up all nuclear weapons prog­rammes, according to the outgoing US national security adviser.

Stephen Hadley urged tough questions over highly enriched uranium (HEU), while encouraging Barack Obama, the president-elect, to keep faith with the six-party framework to deal with North Korea.

“We strongly believe that there is an undetermined amount of highly enriched uranium in North Korea,” Mr Hadley told the Financial Times. “It got there either because it was manufactured there or because it was imported – we don’t know which. But either way, it’s got to be explained.”

North Korea has had a plutonium-based nuclear programme for more than two decades and tested its first nuclear device in 2006, but the US believes the secretive country has also tried to develop a second, uranium-based programme.

Without giving details, Mr Hadley said there was evidence that Pyongyang had continued to experiment with HEU since the US first confronted Kim Jong-il’s regime over the issue in 2002. “That, of course, gives us real concern,” he said.

He predicted that Pyongyang would seek fresh bilateral talks with the new administration but argued that the six-party process still offered the best chance of a lasting settlement. The administration of George W. Bush launched the six-party talks – involving the US, North and South Korea, China, Japan and Russia – in 2003 to apply multilateral pressure on Pyongyang.

North Korea agreed in 2005 to denuclearise in return for economic and political incentives but progress towards concluding the deal has been slow.

Mr Hadley’s remarks came days after Mr Bush, the outgoing president, voiced renewed concern over North Korea’s suspected HEU programme at his farewell press conference.

However, the possible existence of an HEU programme in North Korea remains a source of intense debate in intelligence circles. Michael McConnell, director of national intelligence, on Friday said the US ­intelligence community‘s consensus view – that it believes with “moderate” confidence that North Korea had an active HEU programme – had not changed.

Some critics of Mr Bush have suggested that the White House is highlighting fears over HEU before leaving office to justify its decision to confront North Korea in 2002 – a move that led to the collapse of a Clinton-era agreement that had frozen Pyongyang’s more advanced plutonium programme.

Over the past year, the US has downplayed concerns about HEU as it focused on getting Pyongyang to disclose and verify its plutonium programme. In recent weeks, however, government scientists have determined that particles found on documents and aluminium tubes received from North Korea as part of the verification process contained HEU.

Some sceptics have suggested that the WhiteHouse is highlighting concerns over HEU before leaving office to justify its decision to confront North Koreain 2002 – a move that led to the collapse of a Clinton-era agreement that had frozen Pyongyang’s more advanced plutonium programme.

Over the past year, the US has downplayed concerns about HEU as it focused on getting Pyongyang to disclose and verify its plutonium programme. In recent weeks, however, government scientists have determined that particles found on documents and aluminium tubes received from North Korea as part of the verification process contained HEU.

Some analysts have speculated that the HEU came from a centrifuge that Pakistan provided North Korea in the 1990s. Intelligence analysts have dated the HEU to three and a half years, prompting the Defence Intelligence Agency to argue that it must have been produced in North Korea because there has been no known cooperation between Pakistan and North Korea since 2002.

A senior official stressed that the time frame was not a “smoking gun” because the analysis had only shown that the particles were exposed to the atmosphere in North Korea in 2005. He said the uranium could have been enriched years earlier outside North Korea.

Addressing other issues, Mr Hadley said the US stood by Pervez Musharraf, the former Pakistani president, until his resignation because Mr Bush feared the country could follow a similar path to Iran if the US abandoned its ally.

“The president had in mind the Shah of Iran …and said, ‘I’m not going to do that’,” Mr Hadley said, referring to the US-backed leader whose overthrow in 1979 helped hand power to Islamic extremists in Tehran.

Mr Hadley added the election of a black president with a multicultural background had increased faith in US democracy in the Middle East, even claiming it would advance Mr Bush’s “freedom agenda” in the region.

“It was a convincing display that …contrary to the view of many, our democracy and electoral process was not a sham.”

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