SEOUL - North Korea said it would restart a nuclear reactor to feed its atomic weapons programme, in its clearest rebuff yet of UN sanctions at the heart of soaring tensions on the Korean peninsula.

The announcement was the latest in a series of threats and gestures from Pyongyang that have prompted the deployment of nuclear-capable US B-52s, B-2 stealth bombers and a US destroyer to South Korea on "deterrence" missions.

A Pyongyang government nuclear energy spokesman said the move would involve "readjusting and restarting" all facilities at the Yongbyon nuclear complex, including a uranium enrichment plant and a five-megawatt reactor.

The aim was to "bolster the nuclear armed force both in quality and quantity" as well as solve "acute" electricity shortages, the spokesman was quoted as saying by the official KCNA news agency.

The North shut down the reactor in July 2007 under a six-nation aid-for-disarmament accord. The following summer it destroyed the cooling tower.

The reactor was the sole source of plutonium for the nuclear weapons programme, and experts say the North's standing plutonium stockpile is only enough for four to eight bombs.

However North Korea revealed it was enriching uranium at Yongbyon in 2010 when it allowed foreign experts to visit the centrifuge facility there. It insisted at the time that it was solely low-level enrichment for energy purposes.

The mention of "readjustment" by the energy spokesman will fuel concerns that it will be transformed, if indeed it has not been already, into a facility for openly producing weapons-grade uranium.

The South Korean foreign ministry said the latest statement was "very regrettable".

"The North should honour agreements and promises that have been made in the past... we will closely monitor the situation," a ministry spokesman said.

Many observers believe the North has been producing highly-enriched uranium in secret facilities for years, and that the third nuclear test it conducted in February may have been of a uranium bomb.

Its previous tests in 2006 and 2009 were both of plutonium devices.

US intervention

The Korean peninsula has been caught in a cycle of escalating tensions since the February atomic test, which followed a long-range rocket launch last December.

Subsequent UN sanctions and annual South Korea-US military exercises have been used by Pyongyang to justify a wave of increasingly dire threats against Seoul and Washington, including warnings of missile strikes and nuclear war.

On Monday the United States said a destroyer, the USS Fitzgerald, had been deployed to South Korea's southwest coast in what a US defence official described as "a prudent move" given the current tensions.

North Korea has already threatened to strike the US mainland and US bases in the Pacific in response to the participation of B-52s and stealth bombers in the joint drills with South Korea.

But the tough talk has yet to be matched by action on the ground.

"Despite the harsh rhetoric we're hearing from Pyongyang, we are not seeing changes to the North Korean military posture, such as large-scale mobilisations and positioning of forces," said White House spokesman Jay Carney.

Tuesday's nuclear announcement will, however, be serious cause for concern.

It followed a meeting of the ruling party's top leadership on Sunday, at which young leader Kim Jong-Un stressed the importance of upgrading the country's nuclear arsenal.

"Modernisation of the atomic energy industry is a key to... developing the technology to produce miniaturised, lighter nuclear weapons to a whole new level," Kim said.

The next day the annual session of the North's parliament adopted a law enshrining North Korea's status as a nuclear weapons state.

A basic uranium bomb is no more potent than a basic plutonium one, but the uranium enrichment path holds various advantages for the North, which has substantial deposits of uranium ore.

It also poses a significant proliferation risk.

Highly enriched uranium is the easiest fissile material with which to make a crude bomb, and the technical know-how and machinery for enriching uranium is more readily transferred and sold.