According to US intelligence chief James Clapper, North Korea has restarted a plutonium production reactor that could provide a stockpile for nuclear weapons.
James Clapper also said North Korea had taken steps towards making an intercontinental ballistic missile system.
His announcement comes days after North Korea launched a long-range rocket, which critics say is a test of banned missile technology.
In September 2015, Pyongyang said its main nuclear facility at Yongbyon had resumed normal operations.
The reactor there has been the source of plutonium for its nuclear weapons program.
“We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor,” James Clapper told the Senate Armed Services Committee.
“We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor’s spent fuel within a matter of weeks to months.”
According to James Clapper, Pyongyang was also committed to developing a long-range, nuclear-armed missile “capable of posing a direct threat to the United States”.
James Clapper said it had publicly displayed a mobile intercontinental ballistic missile system and had taken “initial steps toward fielding this system, although the system has not been flight-tested”.
Experts have said that, when fully operational, the Yongbyon reactor could make one nuclear bomb’s worth of plutonium per year. About 4kg of plutonium is needed in order to make a bomb that would explode with a force of 20 kilotons.
Pyongyang has pledged several times to stop operations at Yongbyon and even destroyed the cooling tower in 2008 as part of a disarmament-for-aid deal.
In March 2013, following a row with the US and with new UN sanctions over a third nuclear test, it vowed to restart all facilities at Yongbyon.
Six-nation talks involving South Korea, the US, China, Japan and Russia aimed at ending the North’s nuclear programme have been stalled since 2009.
North Korea says 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 the claim.