Pakistan says it has busted a major prostitution ring smuggling Christian girls to China for fake marriages, a week after Human Rights Watch said it was seeing a worrying and growing trend of such reports.

Parents in poor Christian families are being pressured to offer up their daughters to Chinese husbands at a rate that has boomed since October last year, activists said, with members of the clergy complicit in the smuggling operations and sometimes helping brokers scout for young brides.

The practice of smuggling “brides” to China from other Asian countries has become well established in recent years; a hangover from Beijing’s one-child policy that skewed the gender balance of the population in favour of men.

Human Rights Watch says that after Vietnam, Laos and North Korea were targeted in the past, desperation has seen brokers expand into Pakistan as well. In a statement, the group called on China and Pakistan to act in the face of “increasing evidence that Pakistani women and girls are at risk of sexual slavery in China”.

Christian community leaders, activists, Pakistani officials and smuggled brides themselves spoke to the Associated Press about a well-oiled system that, by one activist’s estimate, has seen 750 to 1,000 girls married off in the past six months or so.

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Young Chinese men would pay on average $3,500 (£2,700) to $5,000, including fees to brokers, members of the clergy and to parents who, in Pakistan’s patriarchal society, would otherwise have to cover the cost of their daughters’ weddings – and a dowry to boot.

They are told that the buyers are wealthy Chinese men who have recently converted to Christianity, but in almost all cases this was found not to be true.

“It is all fraud and cheating. All the promises they make are fake,” said Muqadas Ashraf, who was 16 when her parents married her off to a Chinese man.

Muqadas told the AP her husband turned out to be abusive and they lived “in a small house, just one room and a bedroom”. She said he rarely let her out of the house on her own.

He forced her to undergo a battery of medical tests that later she found were attempts to determine why she was not yet pregnant. On Christmas Eve, when she pressed him to take her to church, he slapped her and broke her phone, she said.

“I don’t have the words to tell you how difficult the last month there was,” said Muqadas. “He threatened me.”

Finally, he agreed to send her home after her family said they would go to the police. Five months after her “wedding”, Muqadas was back in China, pregnant and filing for divorce.

Muqadas’ mother Nasreen said she was promised about $5,000, which included the cost of the wedding and her daughter’s wedding dress. “But I have not seen anything yet,” she said.

“I really believed I was giving her a chance at a better life and also a better life for us,” Nasreen said.

On Monday, police in Pakistan moved to act following a surveillance operation that included attending what authorities said was a fake wedding. They arrested eight Chinese nationals and four Pakistanis, said Jameel Ahmad, a top official at Pakistan’s Federal Investigation Agency (FIA), which deals with human trafficking.

“We busted the gang after the FIA received information about increasing smuggling of Pakistani women to China where they are thrown into prostitution,” Ahmad told the Reuters news agency.

He said the prostitution ring was just one of several gangs that were believed to be in operation, all mainly targeting members of Pakistan’s Christian minority.

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Last month the Chinese embassy in Islamabad said it was cooperating with Pakistani law-enforcement agencies to crack down on illegal matchmaking. It branded reports that the women were being trafficked for the sale of their organs as “misleading and groundless”.

But Aslam Augustine, the human rights and minorities minister in Pakistan’s Punjab province, told AP the Chinese government and its embassy in Pakistan were turning a blind eye to the practice by unquestioningly issuing visas and documents. China’s foreign ministry denied this, saying it had a “zero tolerance” approach to illegal international marriage agencies.

“This is human smuggling,” said Mr Augustine. “Greed is really responsible for these marriages ... I have met with some of these girls and they are very poor.”