BANGKOK (Thomson Reuters Foundation) - The trafficking of women to China for sex work or forced marriage is expanding across Vietnam, an expert said, predicting that more Vietnamese women will be trafficked across South-east Asia as the region moves towards freer trade.

Seventy per cent of trafficking victims in Vietnam are women and girls, with the bulk sent to China as brides, sex workers or factory workers, while others are forced into sex work or marriage in Malaysia, or into brothels in Cambodia, said Ms Mimi Vu of the Pacific Links Foundation.

In the past, most women trafficked into China were from northern Vietnam, near the Chinese border. Over the past year, the trade has expanded south, with officials finding more victims from the Mekong Delta region, said Ms Vu, head of advocacy at Pacific Links, a California-based development organisation.

"Traffickers are willing to take the risk of trafficking victims all the way through Vietnam into China," Ms Vu said in a recent interview in Bangkok, adding that this means the "trade for brides is becoming more lucrative".

China is a destination for trafficked brides because of its shortage of women as a result of its strict one-child policy introduced in the late 1970s. The country last year eased restrictions to allow all couples to have two children.

Ms Vu said economic growth in Vietnam and South-east Asia has created more opportunities for criminals to dupe workers, including domestic migrants who leave rural areas for jobs in industrial zones, with little knowledge about human trafficking.

The 10-member Asean's shift towards a formal community with freer movement of trade and capital would increase trafficking risks, she said.

Already, an increasing number of factories in Malaysia are staffed by Vietnamese workers, and traffickers are "piggybacking" on that migration and tricking people with promises of jobs in electronics factories, she said.

"In reality, they get there, their passports are taken away, and they're forced into prostitution or into being brides," Ms Vu said, noting that many women also end up resold into factories. "You could be trafficked as a bride, give birth to a son, and then sold off to a brothel or a factory, or sold off to another family to give birth again... Her value as a commodity is almost limitless."

In 2013, Pacific Links developed a counter-trafficking programme for factory owners, managers and workers, with support from businesses complying with the California Transparency in Supply Chains Act.

The law, which came into effect in 2012, requires large retailers and manufacturers doing business in the US state to publicly disclose measures taken to eradicate forced labour and human trafficking from their supply chains.

Pacific Links' Factory Awareness to Counter Trafficking (FACT) programme trains managers on international and local laws, as well as the traffickers' tactics to trick workers on the factory floor and in the industrial zone.

It teaches workers what human trafficking is, what to ask if they are approached, and how to report cases.

The FACT programme is now expanding after the passage last year of Britain's Modern Slavery Act, which requires companies operating in Britain with a turnover of US$51 million or more to disclose what they have done to ensure slave labour is not in their supply chains.

Pacific Links has trained 9,000 workers so far, and aims to at least double that number by the end of 2016.

Ms Vu was a speaker for a panel at Trust Forum Asia, an event in Singapore last month on trafficking and slavery hosted by the Thomson Reuters Foundation.