North Korea has restarted a plutonium production reactor that could provide a stockpile for nuclear weapons, US intelligence chief James Clapper says.
He also said the North had taken steps towards making an intercontinental ballistic missile system.
It comes days after the North launched a long-range rocket, which critics say is a test of banned missile technology.
Last September 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 programme. The North carried out its fourth nuclear test in January.
"We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor," Mr 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."