North Korea has expanded its nuclear programme and restarted a plutonium reactor at its main nuclear facility, the United States' intelligence chief said.
"Pyongyang continues to produce fissile material and develop a submarine-launched ballistic missile," Director of National Intelligence James Clapper told a US Senate committee late Tuesday.
North Korea's recent missile and nuclear activity has alarmed its regional neighbours, in particular South Korea and Japan, and provoked warnings from the US, China, and the United Nations.
Pyongyang is also trying to develop a mobile, long-range, nuclear missile system, but it had "not been flight-tested," Clapper told lawmakers as he delivered the annual Worldwide Threat Assessment report.
The restart of the plutonium reactor at the Yongbyon facility meant that Pyongyang "could begin to recover plutonium from the reactor’s spent fuel within a matter of weeks to months," the threat assessment said.
It would take a year to recover enough plutonium to make a bomb, South Korea's Yonhap News Agency reported.
On Sunday, communist North Korea, led by Kim Jong Un, launched a long-range ballistic missile and sent a satellite into orbit. On January 6 it detonated a nuclear device in its fourth nuclear test since 2006.
On Monday, the South Korean military fired warning shots at a military vessel from the North that crossed their maritime border.