In a TV report by public broadcaster NDR, South Korean-born US citizen Kim Dong-chul, 67, recounts his former espionage operations, arrest and the abuse and torture he suffered behind bars.

"I approached military officers and scientists who I knew needed money," Kim says in the programme, showing crooked fingers that he claims were broken by soldiers' boots during his interrogation.

Kim Dong-chul was one of three American detainees freed by Pyongyang in May 2018, in the lead-up to the first summit between US President Donald Trump and North Korean leader Kim Jong Un.

The ex-prisoner has since told South Korean media that he used to gather information for the country's National Intelligence Service and the US Central Intelligence Agency.

The businessman and former Christian missionary had become a trusted insider in North Korea, where from 2001 he ran a hotel in the Rason special economic zone near the China and Russia borders.

He was arrested in October 2015 after he reportedly received a USB stick containing nuclear-linked data and other military information from a former North Korean soldier.

In April 2016 he was sentenced to 10 years' hard labour for subversion and espionage.

In the TV programme, he reports that after the 2011 death of former leader Kim Jong Il, he was recruited by a CIA agent in South Korea.

"After Kim Jong Il's death, there were many rumours about possible successors and the future course of the country," the presumed ex-agent tells the NDR.

Agreeing to gather intelligence on the regime and its arms programmes, he was equipped with a wristwatch that featured a concealed camera and an eavesdropping device he could wear inside his ear.

He says he photographed ships of which the CIA previously had only inaccurate satellite images.

Kim Dong-chul also shows a photo of an ingot of high-purity zinc, a substance used in weapons technology, from the former Soviet Union, which he says he purchased from a North Korean nuclear scientist.

NDR on Friday pre-released details from the programme that was due to screen at 1720 GMT Sunday.

The broadcaster said its inquiry to the US administration about Kim Dong-chul's alleged CIA activities had so far remained unanswered.