North Korea says its main nuclear facility, the Yongbyon complex, has resumed normal operations.
The country was improving its nuclear weapons “in quality and quantity”, state-run news agency KCNA reported.
Yongbyon’s reactor was shut down in 2007 but Pyongyang vowed to restart it in 2013, following its third nuclear test and amid high regional tensions.
The reactor has been the source of plutonium for North Korea’s nuclear weapons programme.
Experts believe that if re-started, the reactor could potentially make one bomb’s worth of plutonium per year.
The announcement about Yongbyon is the first official confirmation from North Korea that it has restarted operations there.
A US think-tank said earlier this year that satellite images suggested that work had commenced at the plant.
Analysis: Stephen Evans, BBC News, Seoul
North Korea holds a big celebration in just under a month – the 70th anniversary of the foundation of its Workers Party. It is building up to that with two significant announcements.
There is the revelation that the country’s main nuclear reactor, which could be used for producing material for atomic weapons, is operating again. But North Korea also says it is ready to launch a rocket with a satellite. Other countries fear this is code for testing the kind of long-range missile that could potentially reach the United States with a nuclear warhead.
This does not mean that North Korea has the ability to do that but it does indicate that there is no let-up in its nuclear ambitions, and that it is intent on making progress towards a viable long-range nuclear missile.
Rocket launch plans
KCNA also said on Tuesday that the North was ready to face US hostility with “nuclear weapons any time”.
However, the full scope of North Korea’s nuclear capabilities is unclear.
Pyongyang claims it has made a device small enough to fit a nuclear warhead on to a missile, which it could launch at its enemies.
However, US officials have cast doubt on this claim and experts say it is difficult to assess the progress North Korea has made on miniaturisation.
North Korea has made threats against its neighbours and the US before, often to coincide with annual joint military exercises held by South Korea and US forces.
The two Koreas remain technically at war, because the 1950-1953 war ended in an armistice, not a peace treaty.
China, which is North Korea’s main ally and trading partner, said on Tuesday that it hoped “relevant parties” would respond with “caution and refrain from taking actions that may elevate tension”.
Yongbyon nuclear complex
- North Korea’s main nuclear facility, believed to have manufactured material for previous nuclear tests
- Reactor shut down in July 2007 as part of a disarmament-for-aid deal
- International inspectors banned in April 2009 when North Korea pulled out of disarmament talks
- 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
- Reactor restarted in 2013, the same year North Korea conducted a nuclear test. Became dormant in August 2014
- Experts believe that reactor could make one bomb’s worth of plutonium per year
- Nuclear test based on uranium device would be harder to monitor than plutonium