Satellite images suggest North Korea may have doubled uranium enrichment capacity at its main Yongbyon nuclear complex, a US think-tank that tracks the North's nuclear weapons programme said Thursday

The Institute for Science and International Security (ISIS) said the images showed the building housing the gas centrifuge plant at Yongbyon had been expanded to twice its original size over the past four months.

When the North revealed the existence of the facility to visiting scientists in 2010, it contained 2,000 centrifuges for enriching uranium.

Assuming that the recent expansion would allow the doubling of that number to 4,000 centrifuges, the facility could produce as much as 68 kilograms of weapons-grade uranium a year -- enough for three nuclear bombs with a little left over, ISIS said in a report.

Given the highly covert nature of Pyongyang's nuclear programme, the ISIS acknowledged that its analysis relied on a large number of hypotheticals.

"It is not even known if North Korea intends to produce weapon-grade uranium in this facility," it said.

When it unveiled the centrifuge plant in 2010, Pyongyang stressed that it was devoted to low-level uranium enrichment for energy purposes.

But in April, at the height of a surge in military tensions with South Korea, the North announced it was "readjusting" its Yongbyon facilities, as well as restarting a five-megawatt nuclear reactor it shut down in 2007 under a six-nation aid-for-disarmament accord.

The announcement stipulated that the move was aimed at feeding its nuclear weapons programme.

The ISIS report noted that assessing the North's uranium enrichment capacity was "fraught with uncertainty" -- especially given suspicions of other hidden centrifuge facilities around the country.

As a result, the ISIS estimated that North Korea could have produced enough weapons-grade uranium in 2012 for anywhere from 0-13 nuclear weapons.

North Korea conducted its third nuclear test in February this year. International observers have been unable to confirm whether it was a plutonium device -- as used in its two previous tests in 2006 and 2009 -- or a uranium one.