Media playback is unsupported on your device Media caption Williamson: "Most people believe North Korea does not want all-out war"

North Korea says it will restart all facilities at its main Yongbyon nuclear complex, including a reactor mothballed in 2007.

In a statement, it said the move would bolster North Korea's nuclear forces "in quality and quantity".

The move is the latest in a series of measures by Pyongyang in the wake of its third nuclear test in February.

It has been angered by the resultant UN sanctions and joint US-South Korea annual military drills.

In recent weeks the communist state has issued a series of threats against both South Korean and US targets, to which the US has responded with high-profile movements of advanced aircraft and warships around the Korean peninsula.

Analysis North Korea's announcement effectively undoes international efforts to constrain its nuclear programme. But restarting the reactor at Yongbyon will take time. Cooling systems have to be re-installed, the reactor fuelled and so on. It could be six months to a year before the reactor is up and running. This will open up a new source of plutonium for North Korea's nuclear weapons programme. Pyongyang's references to its highly-enriched uranium activities are puzzling experts and are less clear. This provides an alternative basis for a nuclear weapon. But nobody knows how many secret enrichment plants North Korea may have or the level to which they may already be enriching nuclear material. So in many ways its back to square one in terms of nuclear diplomacy. But six years have passed since 2007 and in the intervening period North Korea's missile capabilities - the means by which it might eventually seek to deliver a nuclear warhead - have improved significantly.

A South Korean foreign ministry spokesman said that if true, the North Korean move would be "highly regrettable".

Chinese foreign ministry spokesman Hong Lei called for restraint from all sides to resolve the "complex and sensitive" situation.

'Adjust and alter'

The reactor at Yongbyon - which was the source for plutonium for North Korea's nuclear weapons programme - was closed in July 2007 as part of a disarmament-for-aid deal.

The cooling tower at the facility was later destroyed, but then the disarmament deal stalled.

Part of the reason the agreement fell apart was because the US did not believe Pyongyang was fully disclosing all of its nuclear facilities - a suspicion later bolstered when North Korea unveiled a uranium enrichment facility at Yongbyon to US scientist Siegfried Hecker in 2010.

While it appeared to be for electricity generation purposes, Mr Hecker said the facility could be readily converted to produce highly-enriched uranium for bombs.

The statement, carried by KCNA news agency, was attributed to a spokesman for the General Department of Atomic Energy.

Yongbyon nuclear complex North Korea's main nuclear facility; thought to have produced the material for 2006 and 2009 nuclear tests

Reactor shut down in July 2007 as part of a disarmament-for-aid deal; Cooling tower dismantled in 2008

IAEA inspectors banned in April 2009 when North Korea pulled out of disarmament talks

Experts believe that, if re-started, reactor could make one bomb's worth of plutonium per year

A uranium enrichment facility was revealed in 2010. An American nuclear scientist said centrifuges appeared to be primarily for civilian nuclear power, but could be converted to produce highly enriched uranium bomb fuel

Nuclear test based on uranium device would be harder to monitor than plutonium

The department had decided "to adjust and alter the uses of the existing nuclear facilities" including "readjusting and restarting all the nuclear facilities in Nyongbyon [Yongbyon] including uranium enrichment plant and 5MW graphite moderated reactor".

The work would be put into practice without delay, the statement said. It also attributed the move to the need to generate more electricity.

In a November 2010 report following his visit to Yongbyon, Siegfried Hecker said based on what he saw he believed North Korea could "resume all plutonium operations within approximately six months" at Yongbyon if so inclined.

The reactor can produce spent fuel rods that can be made into plutonium - which experts believe North Korea used for its nuclear tests in 2006 and 2009. It is not clear whether plutonium or uranium was used in the February test.

The reference to altering nuclear facilities could suggest that North Korea may begin openly enriching uranium to weapons-grade levels, which would give it a second, faster route to making nuclear weapons, reports the BBC's Lucy Williamson, who is in Seoul.

US warship deployed

Timeline: Korean tensions 12 Dec: North Korea fires three-stage rocket, in move condemned by UN as banned test of long-range missile technology

North Korea fires three-stage rocket, in move condemned by UN as banned test of long-range missile technology 12 Feb: North Korea conducts an underground nuclear test, its third after tests in 2006 and 2009

North Korea conducts an underground nuclear test, its third after tests in 2006 and 2009 11 Mar : US-South Korea annual joint military drills begin

: US-South Korea annual joint military drills begin 19 Mar: US flies B-52 nuclear-capable bombers over Korean peninsula, following several North Korean threats

US flies B-52 nuclear-capable bombers over Korean peninsula, following several North Korean threats 27 Mar: North Korea cuts military hotline with South

North Korea cuts military hotline with South 28 Mar: US flies B-2 stealth bombers over Korean peninsula

US flies B-2 stealth bombers over Korean peninsula 30 Mar: North Korea says it is entering a "state of war" with South Korea

North Korea says it is entering a "state of war" with South Korea 2 Apr: North Korea says it is restarting mothballed Yongbyon reactor How potent are North Korea's threats?

In the weeks since the nuclear test, North Korea's rhetoric has escalated - including multiple threats of attacks on US bases in South Korea and Japan, and on South Korean border islands.

But the US, which has in recent days flown B-52 bombers, B-2 stealth planes and F-22 stealth bombers over South Korea in a show of strength, says it is seeing no signs of increased military activity in North Korea.

"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.

But he said the US was "monitoring the Korean situation very diligently".

Defence officials told reporters that a US Aegis-class warship capable of defending against missile strikes had also been moved to the south-west coast off South Korea.

This would offer "greater missile defence options should [they] become necessary", an official told Reuters news agency.

Reports also suggest the US is moving a sea-based radar platform called SBX-1 into the western Pacific so that it can monitor North Korea.