SEOUL: North Korea said on Tuesday that it intended to restart a mothballed nuclear reactor to feed its nuclear arms programme and hinted it may begin openly enriching weapons-grade uranium.

The announcement was the latest provocation at a time of soaring military tensions and followed a US decision to place a destroyer off the South Korean coast to defend against a possible missile strike from the North.

A government nuclear energy spokesman said the move would involve “readjusting and restarting” all facilities at the North's Yongbyon nuclear complex, including a uranium enrichment plant and a five megawatt reactor.

This was being done with a view to “bolstering the nuclear armed force both in quality and quantity” as well as solving “acute” electricity shortages, the spokesman was quoted as saying by the official KCNA agency.

The North shut down the Yongbyon 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 North's nuclear weapons programme. The country's remaining plutonium stockpile is believed to be enough for four to eight bombs.

North Korea revealed it was enriching uranium in 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 producing weapons-grade uranium.

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

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

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

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 its destroyer, USS Fitzgerald, had moved to South Korea's southwestern coast after taking part in annual military exercises, instead of returning to its home port in Japan.

The deployment was “a prudent move” given the current tensions, a US defence official said, adding that it would offer “greater missile defence options should that become necessary”.

Earlier on Monday, the US military announced it had deployed F-22 Raptor stealth fighters to South Korea as part of the ongoing annual “Foal Eagle”joint military exercise.

North Korea has already threatened to strike the US mainland and US bases in the Pacific in response to the participation of nuclear-capable US B-52 and B-2 stealth bombers in the drill.

But the tough talk has yet to be matched by action on the ground, according to US intelligence.

“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,” White House spokesman Jay Carney said.

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 North Korea's 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.