NEW DELHI: Human traffickers in India are luring village girls into sex slavery with promises of visiting the Taj Mahal, a charity said on Monday, as criminal gangs find new ways to enslave the poor.

Rishi Kant, of anti-trafficking charity Shakti Vahini, said 15 girls and young women were last week rescued from a brothel in the northern city of Agra, home to the 17th century white marble mausoleum which draws millions of tourists annually.

"For many days we were looking for six girls reported missing by their families in West Bengal state and finally managed to trace them to a brothel in Agra's red light district," Kant told the Thomson Reuters Foundation.

"With police, we conducted a raid and found the six girls, and nine others. They had been confined there for two months and forced to have sex with customers. They said they had gone with the trafficker as he promised to take them to see the Taj."

Almost 46 million people are enslaved worldwide - trafficked into brothels, forced into manual labour, victims of debt bondage or even born into servitude - according to the 2016 Global Slavery Index.

Forty per cent, or more than 18 million, are in India. Many are from poor rural regions and lured with the promise of good jobs or marriage but end up sold into prostitution, domestic work, or industries such as brick kilns or textile units.

Kant said interviews with the rescued girls suggested that using the Taj Mahal to lure victims was "an emerging trend" employed by traffickers, who are constantly finding new ways to enslave people as public awareness of the crime spreads.

"There are many initiatives in trafficking-prone areas such as public campaigns to inform people about human trafficking and how these traffickers operate," said Kant.

"But these criminal gangs are also very clever and using new ways to cheat poor illiterate communities. We need to know about these methods and incorporate them into our public awareness work."

The six girls, all aged between 17 and 19 and from villages in the impoverished district of South 24 Parganas on India's eastern border with Bangladesh, were reported missing in March, said Kant.

They were initially taken to Delhi where they were kept in a small flat for six days and then transported to Agra in Uttar Pradesh state - 230 km (140 miles) from the capital - where they were sold to a brothel, he added.

One of girls had managed to phone her family and police traced the mobile phone to Agra. They raided the brothel on Thursday and found the girls hidden in a secret bunker under a bed in one of the rooms. Two of the girls are pregnant.

There were also nine other victims in the brothel.

The victims have been reunited with their families, said Kant, adding that the priority was to ensure they were given medical and psychological support. The brothel owner, a 24-year-old woman, has been arrested. Police are still looking for the traffickers, added Kant.