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 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 started at the plant.
KCNA 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.
Analysis: Stephen Evans, BBC News, Seoul
Two big claims have been made as the country prepares for the 70th anniversary of the foundation of the ruling Workers Party. Firstly, it said the nuclear facility producing material for bombs is fully operational.
Secondly, Pyongyang has indicated it will launch a rocket soon to carry a satellite into space - but the US and South Korea believe it would be a test of a long-range military missile.
Most reliable observers think North Korea already has nuclear material for 12 or so atomic bombs, but may not be able to make those bombs small enough to go on a rocket. Nor has it shown it can send a missile as far as the US (though it has ample ability to hit South Korea).
If it emerged that its rocket technology really had advanced enough to hit California, that would change the calculation completely.
North Korea is indicating it is near that stage, but there is no evidence that this is more than sabre-rattling right now.
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
- In 2013, North Korea said it would restart the nuclear reactor, the same year it conducted a nuclear test. It is believed to have shut it down for a period in 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