North Korea has resumed normal operations at the Yongbyon complex, the country's main nuclear facility, the head of the North's Atomic Energy Institute (AEI) said.
The confirmation on Tuesday came a day after suggestions by the chief of the national space agency of a possible satellite rocket launch next month.
The nuclear reactor is seen as its main source of weapons-grade plutonium and the move raised the second red flag in 24 hours over its nuclear and ballistic missile programs.
Analysts said the two statements were clearly coordinated and partially aimed at raising tensions to place North Korea firmly on the agenda of an expected US-China summit later this month.
In an interview with the North's official KCNA news agency, the director of the AEI said all facilities at the Yongbyon nuclear complex, including the five-megawatt reactor, had "started normal operations".
North Korea mothballed the Yongbyon reactor in 2007 under a six-nation aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.
When fully operational, the reactor is capable of producing around six kilos of plutonium a year - enough for one nuclear bomb, experts say.
North Korea said on Tuesday in state media that the plutonium and highly enriched uranium facilities at its main Nyongbyon nuclear complex have been "rearranged, changed or readjusted and they started normal operation," and that its scientists had improved "the levels of nuclear weapons with various missions in quality and quantity."
A day earlier, the North's aerospace agency said it is ready to launch new satellites aboard long-range rockets as part of celebrations of next month's ruling communist party anniversary.
The world will "clearly see a series of satellites soaring into the sky at times and locations determined" by the Workers' Party, an unidentified director at the North's National Aerospace Development Administration director was quoted as saying by state media.
North Korea has said its satellite launches are peaceful and meant for weather observation, but the West considers them covers for banned tests of long-range missiles.
Analysts in Seoul said the rocket launch announcement suggests an imminent nuclear test. "A nuclear test without a long-range rocket test is not effective," said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University. "The vicious circle is being repeated."
After several failures, North Korea put its first satellite into space with a long-range rocket launched in late 2012.
The UN said it was a banned test of ballistic missile technology and imposed sanctions.
Experts say that ballistic missiles and rockets used in satellite launches share similar bodies, engines and other technology.
Source: AP And AFP