About a year ago, Saraswati Kabiraj (name changed), a 15-year-old girl from Choto Sehara in the border state of West Bengal, eloped with Piklu Mondol, a Bangladeshi who, according to Saraswati's family, often visited India illegally.

Lured by the promise of marriage, Saraswati crossed into Bangladesh. A few days later, Piklu sold her to a brothel along the border, her family said.

Unable to get help from the police, they brought the case to a human rights group, Banglar Manabadhikar Suraksha Mancha (MASUM), which reported it to the Asian Human Rights Commission.

They have heard nothing from her since.

Several such cases of Indo-Bangladeshi human trafficking are pending; many more are never reported. The 4,024-km border is relatively porous and cross-border movement is easy.

"Trafficking in girls and women happens in both directions, depending on the going prices, which in turn depend on the quality of girls and also on how strict the officer on duty is," Indrani Sinha, chairwoman of Sanlaap, a Kolkata-based NGO that runs shelters for rescued girls, told Khabar South Asia.

India's Border Security Force (BSF) and Border Guards of Bangladesh (BGB) are in charge of patrolling the Indo-Bangladesh border and preventing smuggling and human trafficking. But local people say border guards are often party to the crime, according to NGOs that work to prevent it.

BSF authorities did not respond to Khabar South Asia's request for comment.

Most Bangladeshi women who work as prostitutes in India are illiterate, were abandoned by their husbands, and have children to raise. Many were duped by members of their own community on promises of marriage or jobs. Some are actually married and the husbands are the ones who sell them off.

Some women travel to larger Indian cities like Delhi and Mumbai as maids but end up becoming prostitutes in cities, smaller border towns or along highways. Frequent raids on the shanties and brothels in Delhi weed out illegal immigrants, mostly Bangladeshis.

About 500 girls are rescued from Delhi and Mumbai every year, but rehabilitation is usually more difficult than rescue, Roy said.

Rescued women often return to the profession or settle down in the border region again when they are released, since there are not enough shelters, training centers or job alternatives to help them do otherwise.

Sometimes the rescued women have grown accustomed to their life and do not want to return to the village and to poverty.

According to Sanjog, a Kolkata-based NGO that studies the lives of trafficked women along the Indo-Bangladesh and Indo-Nepal borders, some women in the sex trade manage their finances well. Many have bank accounts and insurance policies. They eat better than they did in the village, send their children to school and get advice from AIDS prevention centers.

Case studies and anecdotal evidence reported by NGOs are the only data sources for human trafficking in this region, which has emerged as South Asia's trafficking hub.

It is relatively easy for Bangladeshis to enter India, get valid mobile phone connections and ration cards, said Amiya Kumar Samanta, former Director General of West Bengal Police.

Traffickers have political clout thanks to the money involved, which is why few cases get registered or investigated.

The current number of trafficked women and girls in India is unknown, but a 2004 Indian government study cited in the International Journal for Equity in Health put the figure at 2.8 million.

"The Asian country has emerged as an international supplier of trafficked women and children to the Gulf States and South East Asia, as well as a destination country for women and girls trafficked for commercial sexual exploitation from Nepal and Bangladesh," the journal said in a 2008 article, "Sexual Slavery without Borders: Trafficking for Commercial Sexual Exploitation in India".

An estimated 800 000 women and children are trafficked each year across international borders, the study said.