A shortage of brides in China is putting young women in neighbouring countries at risk of being trafficked

When Kiab turned 16, her brother promised to take her to a party in a tourist town in northern Vietnam. Instead, he sold her to a Chinese family as a bride.

The ethnic Hmong teenager spent nearly a month in China until she was able to escape from her new husband, seek help from local police and return to Vietnam.

"My brother is no longer a human being in my eyes – he sold his own sister to China," Kiab, whose name has been changed to protect her identity, said at a shelter for trafficking victims in the Vietnamese border town of Lao Cai.

Vulnerable women in countries close to China – not only Vietnam but North Korea, Laos, Cambodia and Burma – are being forced into marriages in the land of the one-child policy, experts say.

China suffers from one of the worst gender imbalances in the world as families prefer male children. As a result, millions of men cannot find Chinese brides – a key driver of trafficking, according to rights groups.

The Lao Cai shelter is home to a dozen girls from various ethnic-minority groups. All say they were tricked by relatives, friends or boyfriends and sold to Chinese men as brides. "I had heard a lot about trafficking. But I couldn't imagine it would happen to me," Kiab said.

Since trafficking is run by illegal gangs and the communities involved are poor and remote, official data is patchy and probably underestimates the scale of the problem, according to experts. But rights workers across south-east Asia say they are witnessing systematic trafficking of women into China for forced marriages.

"This problem has largely been swept under the rug by the Chinese authorities," said Phil Robertson, deputy Asia director at Human Rights Watch, in New York.

Vietnamese girls are sold for up to $5,000 (£2,944) as brides or to brothels, said Michael Brosowski, founder and chief executive of the Blue Dragon Children's Foundation, which has rescued 71 trafficked women from China since 2007.

"The girls are tricked by people posing as boyfriends, or offering jobs. Those people do this very deliberately, and for nothing other than greed and a lack of human empathy," he said.

It is likely that many of the girls end up working in brothels, but due to the stigma of being a sex worker they will usually report they were forced into marriage.

Communist neighbours Vietnam and China share a mountainous, remote border stretching 1,350km (839 miles), marked primarily by the Nam Thi river and rife with smuggling of goods of all kinds: fruit, live poultry and women.

"It is mostly women who live in isolated and mountainous areas who are being trafficked across the border, because there is no information for us," said 18-year-old Lang, from the Tay ethnic minority, who walked across the border illegally and was sold to a Chinese family by a friend.

In northern Vietnam, trafficking has become so acute that communities say they are living in fear. "I worry so much about it, as do all the mothers in the villages, but it has happened to a lot of girls already," said Phan Pa May, a community elder from the Red Dao ethnic group. "I have one daughter. She's already married, but I'm worried about my granddaughter. We always ask where she is going, and tell her not to talk on the phone or trust anyone."

Activists working to combat trafficking in Vietnam say police and authorities take the problem "very seriously".

The shelter in Lao Cai opened in 2010 and has helped scores of female victims. "There is nothing at home for these girls, not even enough food to eat," said director Nguyen Tuong Long, referring to the dire poverty that is another key driver. But he said he believes the number of cases is falling.

May Na, from the Hmong ethnic group, was 13 when her uncle took her across the border and forced her to marry a Chinese man. "I could not accept it. They left me at home alone and I climbed over the wall and ran away. I was wandering for more than a day, lost, sleeping in the streets, crying," she said.

Eventually, Na ended up at a police station, but because she spoke neither Chinese nor Vietnamese – only her native Hmong – it took police a month to figure out what had happened and return her to Vietnam.

Now 16, Na, the eldest of five children, is learning Vietnamese at the Lao Cai centre. Her uncle has been arrested, she says, but she has chosen not to return to her family. "I was so sad when I was in China. It was a painful experience for me," she said.

The government has launched education programmes in rural areas near the border, warning young women not to trust outsiders.

Anti-trafficking groups in Vietnam say it is difficult to warn girls of the risks when it is often a family member or friend carrying out the deception. Instead, they want harsher penalties for traffickers – including, for example, prosecutions at local level to raise awareness in villages of potential punishments to deter people from trying.