Washington has suspected for years that Myanmar has a secret nuclear programme supported by North Korea, with witnesses reporting suspicious activity as far back as 2004, leaked memos show.

One cable from the US embassy in Yangon, dated August of that year and released Thursday by the anti-secrecy website WikiLeaks, quoted an unidentified source as saying he saw about 300 North Koreans working at an underground site.

"The North Koreans, aided by Burmese workers, are constructing a concrete-reinforced underground facility that is '500ft from the top of the cave to the top of the hill above'," according to the cable.

"The North Koreans are said to be assembling missiles of unknown origin," it said, adding that the report alone should not been taken as definitive proof or evidence of sizeable North Korean military involvement with the Myanmar regime.

Another memo, also dated 2004, quoted a foreign businessman as saying that he had seen a reinforced steel bar, larger than for just a factory project, being unloaded from a barge in the same area in west-central Magway Division.

The cable said the source had volunteered to an US Embassy Officer that he had heard rumours that a nuclear reactor was being built near the town of Minbu.

It said that while there was no direct evidence of cooperation between Myanmar's generals and Pyongyang, there were increasing reports of alleged sightings of North Korean "technicians" inside the Southeast Asian nation.

US Secretary of State Hillary Clinton warned in July 2009 about possible nuclear links between Myanmar and North Korea, and earlier this year she said a ship from Pyongyang had delivered military equipment to Myanmar.

Myanmar's junta -- which recently held a widely criticised election seen as prolonging military rule -- has dismissed reports of its nuclear intentions and brushed aside Western concerns about possible cooperation with North Korea.

A UN report released last month alleged North Korea is supplying banned nuclear and ballistic equipment to Myanmar as well as Iran and Syria.

And a June documentary by the Norwegian-based news group Democratic Voice of Burma (DVB) said Myanmar was trying to develop nuclear weapons, citing a senior army defector and years of "top secret material."

The DVB documentary gathered thousands of photos and defector testimony, some regarding Myanmar's network of secret underground bunkers and tunnels, which were allegedly built with the help of North Korean expertise.

Robert Kelley, a former director of the International Atomic Energy Agency (IAEA) who inspected the files smuggled out of Myanmar by Sai Thein Win, said in October evidence indicated "a clandestine nuclear programme" was under way.

"This is not a well-developed programme. I don't think it's going very well," he said at the time.

"But if another country steps in and has all of the knowledge, the materials, and maybe the key to some of the things that are plaguing them, including bad management, this programme could really speed up."

Kelley said North Korea was "certainly the country I have in mind".

According to another cable from 2009, a well-placed source in the Myanmar government said General Thura Shwe Mann had visited North Korea in 2008.

But the source backtracked later insisting the talks were only exploratory.

The leaked memos also suggest that key backer China was fed up with the "foot-dragging" of Myanmar's military junta on reform and feared the ruling generals could no longer protect its interests in the country.

"The Chinese can no longer rely on the generals to protect their interests here, and recognise the need to broker some solution that keeps the peace," according to a US cable dated January 2008.