North Korea has allegedly 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," James Clapper, the director of national intelligence, told a US Senate committee late on 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.
Clapper told the senators during his report on the annual Worldwide Threat Assessment that Pyongyang is supposedly trying to develop a mobile, long-range, nuclear missile system - though it had "not been flight-tested".
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The restarting 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, North Korea, led by leader 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.
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