North Korea says it has resumed plutonium production from spent fuel rods and has no plans to stop nuclear tests as long as the United States still "threatens" Pyongyang, Kyodo News reported Wednesday.

The North's Atomic Energy Institute, which has jurisdiction over the country's main atomic complex Yongbyon, told Kyodo it had been producing highly enriched uranium for nuclear arms and power "as scheduled".

"We have reprocessed spent nuclear fuel rods removed from a graphite-moderated reactor," the agency said in a written interview with Kyodo.

The agency did not disclose how much plutonium or enriched uranium the North has produced, Kyodo said.

The type of plutonium suitable for a nuclear bomb typically needs to be extracted from spent nuclear reactor fuel.

In June, the UN's atomic watchdog warned that North Korea could have reactivated the Yongbyon plant for reprocessing plutonium for use in nuclear weapons, following similar warnings from a US think tank.

The director of US National Intelligence, James Clapper, warned in February that the North could begin recovering plutonium from the reactor's spent fuel "within a matter of weeks to months".

North Korea mothballed the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its third nuclear test in 2013. It carried test out a fourth on January 6.

The North's Atomic Energy Institute did not rule out the possibility of further nuclear tests, claiming it had had success in "minimising, making lighter and diversifying" nuclear weapons, Kyodo said.

"Under conditions that the United States constantly threatens us with nuclear weapons, we will not discontinue nuclear tests," the institute said, according to Kyodo.