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.
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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.
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.
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.
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. —BBC