Businesses established by Pyongyang abroad -- including restaurants like this one in Shenyang, China -- are a crucial source of foreign exchange

The 12 North Korean waitresses who defected from China two years ago were tricked into doing so in an operation by the South's intelligence services, their manager told South Korean television in a bombshell revelation.

The high-profile case has long been controversial, with Pyongyang insisting the women had been kidnapped and saying there would be no more reunions of families divided by the Korean War unless they were returned.

Seoul insisted that they had defected of their own free will.

But Heo Gang-il, the manager of the North Korean restaurant in Ningbo where they worked, said he had lied about their final destination and blackmailed them into following him to the South.

Heo told JTBC television he had been recruited by Seoul's National Intelligence Service (NIS) in China in 2014.

Fearing exposure in 2016, he asked his NIS handler to arrange his defection. At the last minute the minder told him to bring his staff too.

"The 12 waitresses did not know where they were going," Heo told JTBC's Spotlight, one of the South's top investigative current affairs programmes.

"I told them we were relocating," he said.

The women only realised their final destination when they arrived outside the South Korean embassy in Malaysia.

When they hesitated to enter the building, one of them told the show, "manager Heo threatened us, saying he will tell security authorities that we watched South Korean TV dramas and we would be executed, or exiled into provinces and our families would also be affected".

"Thinking back, it was all nonsense but back then, I had no other choice," she said.

"If it was possible for me to go home even now, I would like to return to the bosom of my mother," she said.

Campaigners were outraged.

Lawyers for a Democratic Society, an influential group of human rights lawyers which has unsuccessfully been seeking to interview the waitresses, called for a thorough investigation of what they branded a "heinous crime committed by the NIS".

Those responsible should be given "stern punishment" and "the waitresses must be allowed to return home and reunite with their families", it said in a statement.

- 'Need to verify' -

The group's arrival in the South in April 2016 made headlines as the largest group defection for years, while Pyongyang waged a vocal campaign through its state media demanding their immediate return.

After four months of acclimatisation education -- standard for defectors -- the women were released into society, but their whereabouts were kept secret by the NIS and they had made no public comments until the television show late Thursday.

Heo said he was speaking out as rewards he had been promised -- an NIS job and a medal -- had not materialised.

Seoul's unification ministry, which handles relations with the North, said the facts needed to be established.

"There are some new allegations by the manager and some waitresses about how they came to the South and whether they came here on their free will," a spokesman told journalists.

"There is need to verify their allegations," he said, adding the ministry had been unable to contact the 13 itself and relied on information given it by the NIS.

An NIS spokesman said the agency had no immediate comment on the report.

Sceptics suspect the defection may have been staged by the government of now-disgraced conservative president Park Geun-hye in an effort to sway voters ahead of parliamentary elections.

Park, 66, who was removed from office over a massive corruption scandal last year, was convicted of multiple criminal charges including bribery and abuse of power and sentenced to 24 years in prison at a trial last month.

Businesses established by Pyongyang abroad -- including restaurants like this one in Shenyang, China -- are a crucial source of foreign exchange