TOKYO - 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.

Meanwhile, South Korea said Wednesday that North Korea's deputy ambassador to Britain had defected to Seoul, in a rare and major loss of diplomatic face for Pyongyang.

The Unification Ministry said Thae Yong-Ho - the number-two at the North's mission in London - had defected together with his family and they were now in the South Korean capital.

"They are under government protection and are going through necessary procedures with related institutions," ministry spokesman Jeong Joon-Hee told reporters.

Jeong declined to reveal Thae's defection route, citing the diplomatic sensitivities involved for the concerned countries.

"On his reasons for defection, Minister Thae cited disgust with (North Korean leader) Kim Jong-Un's regime, admiration for South Korea's free, democratic system and the future of his family," Jeong said.

Increasingly isolated internationally because of its nuclear weapons programme, North Korea maintains relatively few overseas embassies, and defections by diplomats of Thae's stature are extremely rare.

The last such case was that of the North Korean ambassador to Egypt who defected to the United States in 1997. South Korea' JoongAng Ilbo newspaper, which first reported Thae's defection on Tuesday, said he had been under pressure from Pyongyang to combat growing international criticism of North Korea's human rights record.

North Korean defectors have been making headlines recently, largely due to an unusual group defection in April by a dozen waitresses and their manager who were working at a North Korean-run restaurant in China.

A North Korean army colonel who had handled spying operations on South Korea was announced to have defected last year.

And, in July, an 18-year-old student, who was in Hong Kong for an international maths contest, reportedly sought asylum in the South Korean consulate in the city.

The Unification Ministry said Thae's defection reflected the loss of faith among North Korea's elite in Kim Jong-Un's leadership.

"Awareness that the North Korean regime has reached its limit is spreading and the solidarity of its ruling class is weakening," Jeong said.

Thae was believed to have worked at the embassy in London for 10 years, with one of his main tasks being to counter the image of North Korea as a nuclear pariah state and notorious human rights abuser.

Over the years, nearly 30,000 North Koreans have fled poverty and repression in their country and settled in the South.

But the number of defectors - who once numbered more than 2,000 a year - has nearly halved since Kim Jong-Un took power after the death of his father and former leader Kim Jong-Il in December 2011.

Those who still managed to flee in recent years often had families already settled in the South, or were relatively well-off and well-connected members of the elite in search of better lives.

The highest-ranking defector to come to the South was Hwang Jang-Yop, the North's chief ideologue and former tutor to Kim Jong-Il. He made a high-profile defection via the South Korean embassy in Beijing in 1997 and died in Seoul in 2010.