The confirmation, by the head of the North's Atomic Energy Institute (AEI), raises a further red flag amid growing signs the North may be considering a long-range rocket launch next month in violation of UN resolutions.
In an interview with the North's official KCNA news agency, the institutes's director said all facilities at the Yongbyon nuclear complex -- including a five-megawatt reactor -- had "started normal operations".
North Korea mothballed the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.
Since then, satellite imagery analysis has suggested the partial and intermittent restart of the reactor which, when fully operational, is capable of producing around six kilos (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.
The AEI director, who was not named, said scientists and technicians had been "steadily improving" both the quality and quantity of the North's nuclear deterrent and issued a by-now standard warning to the United States.
"If the US and other hostile forces persistently seek their reckless hostile policy ... (North Korea) is fully ready to cope with them with nuclear weapons any time," he said.
The warning followed strong hints from Pyongyang that it is considering a satellite rocket launch to mark the 70th anniversary of the founding of the ruling Workers' Party on October 10.
The North insists its rocket launches are intended to put peaceful satellites into orbit, while the United States and its allies see them as disguised ballistic missile tests.
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