U.S. intelligence has said North Korea is advancing its nuclear weapons programme following threat to do so.
Director of National Intelligence James Clapper said the North has expanded the size of its uranium enrichment facility at its Nyongbyon nuclear complex.
According to CBS News, Clapper also said the country has restarted a plutonium reactor that was shut down in 2007.
Clapper's written testimony to the Senate Intelligence Committee coincides with the assessments of think tanks that monitor North's nuclear programme using commercial satellite imagery.
South Korean intelligence also said the reactor has restarted.
According to the report, Clapper said the North has taken initial steps toward fielding a road-mobile intercontinental ballistic missile although it remains untested.