North Korea's deputy ambassador in London has defected to the South, according to officials in Seoul.

Thae Yong Ho has apparently arrived with his family in Seoul, making him the highest-ranking diplomat ever to defect to the South.

He defected due to discontent with the regime and for the future of his child, the South's Unification Ministry said.

Jeong Joon-hee, spokesman at the ministry, told a news conference: "They are currently under government protection and relevant institutions are going ahead with necessary procedures as usual."

Jeong declined to give details on the timing of Thae's arrival or his itinerary.

Defections are a source of bitter contention between the rival Koreas, and South Korea doesn't always make high-profile cases public.

North Korea often accuses the South of kidnapping or enticing its citizens to defect.

The defection comes after North Korea responded angrily to a US plan to place an advanced missile defense system in South Korea.

The North has warned of unspecified retaliation and fired several missiles into the sea earlier this month.

Meanwhile, North Korea said it has resumed plutonium production by reprocessing spent fuel rods.

It said hat it has no plans to stop nuclear tests as long as perceived US threats remain, Japan's Kyodo news agency reported.