VIENTIANE (NYTIMES) - For more than two years, Lee Jin-hui, 20, was never allowed to leave a three-room apartment in north-east China. Seven days a week, she had to sit at a computer from noon to 5am, performing sex acts before a webcam for male clients, mostly from South Korea.

In the apartment, Lee and other North Korean women each had to earn about US$820 (S$1,100) a week for the Chinese pimp who bought them from human traffickers. When they failed, they were slapped, kicked and denied food.

"We had to work even when we were sick," Lee said. "I wanted to get out so badly, but all I could do was peek out the window."

Each year, human smugglers take thousands of women seeking to flee North Korea, promising them jobs in China, according to human rights groups and trafficking survivors. But once in China, many of the women are sold to unmarried men in rural towns or to pimps for exploitation in brothels and cybersex dens.

If they are caught running away from traffickers, China sends them back to North Korea, where they face torture and incarceration in labour camps. With nowhere to turn for help in China, they remain trapped in sex slavery.

An estimated 60 per cent of female North Korean refugees in China are trafficked into the sex trade, and increasingly coerced into cybersex, the London-based rights group Korea Future Initiative said in a report in May.

"Girls aged as young as 9 are forced to perform graphic sex acts and are sexually assaulted in front of webcams, which are live-streamed to a paying global audience, many of whom are believed to be South Korean men," the report said.

When she was smuggled out of North Korea in spring 2017, Lee was told she would be waitressing in China. When she arrived, her boss said her job was "chatting" at the computer. Until then, she had never seen a computer. She didn't know what a webcam was. She was 18.

"I thought 'chatting' was some kind of bookkeeping with a computer," said Kim Ye-na, 23, who was smuggled out last November, believing she would pick mushrooms in China. "I never imagined what it would turn out to be." Both Lee and Kim fled their captivity on Aug 15.

Six days later, they arrived in Vientiane, Laos, with a man who was paid US$4,000 to smuggle them across the China-Laos border. Waiting for them was Reverend Chun Ki-won, a Christian pastor from South Korea who funded and orchestrated their rescue.

The women agreed to interviews while in Vientiane, using nicknames they were given on the run to protect their privacy and avoid the North Korean government's possible retaliation against their relatives in the North. Though The New York Times could not independently corroborate some details of their flight, recordings of online conversations between Chun and the women made before their escape supported their accounts.

"Given China's increasing crackdown on undocumented foreigners, locking North Korean women in apartments for cybersex has become a favourite way for human traffickers to exploit them," Chun said. "They drug the women to dull their shame and make them work long hours."

Kim toiled in a jade mine and later joined the unofficial market, selling fruits and South Korean clothes smuggled from China. Lee collected and sold wild herbs.

As they grew up, their home town, Hyesan, and other towns along the narrow river border with China became hunting ground for human traffickers. In 2017, a relative sold Lee.

"I myself wanted to go to China because I heard of girls gone there sending money to their families," said Lee.

After changing hands twice between human traffickers, Lee ended up with a man who held five North Korean women captive in Helong, in north-east China.

Kim, too, wanted a way out. North Korea's leader, Kim Jong Un, had begun cracking down on young merchants in the markets, hoping to drive them towards state-led building projects. A female smuggler whom Kim had befriended agreed to take her to China.

At 4am on Nov 18, the smuggler, her brother and Kim were waiting at the border when a soldier appeared out of the darkness. He told them the way was clear.

The smuggler knew the path well, leading them across the icy, shallow river and through a hole under the border fence. After walking 12 hours through hills, the smuggler dug out a cellphone buried in the ground and made a call.

Hours later, a woman appeared in a car. She handed Chinese cash, a bundle of shoes, clothes and other items to the North Korean smuggler. Kim was being traded.

The woman who bought Kim was also from Hyesan and worked for a sex trafficking ring, managing a dozen webcam women, all from Hyesan, in apartments scattered around Gongzhuling, in north-east China. She said Kim owed her 80,000 renminbi (S$15,500).

"She said I can go to South Korea after working for her three years," Kim said. "I heard that in South Korea, you can live decently if you work real hard. That was all I was asking for." Some of Lee's South Korean clients asked her to do sex acts too dehumanising for her to describe.

"If I refused, they called me dirty trash from North Korea," she said.

Other men took pity on the women. Two of Lee's clients regularly sent cash to her boss so she could get some extra sleep.

Last December, a woman disappeared from Lee's place. The pimp said she had been lured away by organ traffickers and must be dead, leaving the other women terrified.

Kim said only two women were released from her apartment - when both developed tuberculosis. After being severely beaten, two others tried to escape from their sixth-floor apartment by scaling down water pipes. The police soon arrested them, but the pimp refused to pay bribes to stop their repatriation to North Korea. She was making them a lesson for the others.

"She said, 'Remember how much your life here is better than the one you left behind in the North,'" Kim said.

Despite her slavelike condition, Lee never thought of going back to North Korea. Her aim was to get to South Korea and make enough money to smuggle her mother and a younger sister out.

"I kept telling myself, 'Hang in there. When the time comes, you can make it to South Korea,'" she said.

In late 1995, Chun, then a hotelier and not yet a pastor, was on a business trip to Hunchun, a Chinese city on the border with North Korea, when he realised how dire circumstances were. He saw the uncollected bodies of North Koreans fleeing famine, frozen to death while crossing the river border; the Chinese police clubbing child beggars to drive them away; a woman screaming for help as she was kidnapped by two men.

Chun later became a Christian missionary. Since 2000, he has brought 1,200 North Korean refugees in China to South Korea, including many women trafficked into forced marriages. In recent years, however, his Durihana mission in Seoul started receiving anonymous online messages from women trapped in cybersex dens in China, and calls from men who wanted to rescue them.

One such call was from an animal feed deliveryman in South Korea in July.

He sent Kim's boss 15 million South Korean won (S$17,000) to buy her freedom. But the smuggler who promised to take Kim to South Korea sold her to a Chinese man in his 50s. The South Korean man sent another 15 million to Kim's original boss to free her from the forced marriage. By then, he realized he had been duped.

Around the same time, Chun got a call from a man who wanted to help Lee. She also got surprising news: The woman said to have been kidnapped by organ traffickers contacted her through a webcam site. She had jumped from their third-floor apartment and now lived in South Korea.

Chun contacted Lee and Kim, pretending to be a client.

The woman who had escaped helped Chun find Lee's neighborhood. Kim memorised the telephone number of a nearby restaurant that her boss once took her to. By peeking out the windows, Lee and Kim identified as many landmarks as possible to help Chun pinpoint their locations on Google Earth.

Chun then sent seven volunteers to China, including two trafficking survivors.

On Aug 15, one team waited in a taxi outside Kim's apartment and followed her, another girl and their boss when a sudden water outage forced them to go out for food. Kim pretended to be sick on their way back, vomiting on the sidewalk and running into a public toilet. When the boss entered another stall, Kim rushed out into the rescuers' taxi and it sped away.

Asked what she had wanted most, Kim said, "standing outside in a pouring rain." But days after leaving China, she still had nightmares about being on the run, someone chasing her close behind.

On that same August day in Helong, Lee slipped out of her room while her Chinese pimp was out for drinks. Through the window of the living room, she saw an air mattress and a rescuer beckoning. She climbed out, then hesitated.

"The height was terrifying," she said. "But it was the only way out." She jumped.

In late August, a black van stopped across the street from the South Korean embassy of a South-east Asian country where defectors can apply for asylum. Holding Chun's hands, Lee and Kim got out and crossed the road, walking their last yards to freedom. The steel gate slid open, and the girls stepped inside.