Mumbai: A mobile phone app is the latest tool for campaigners seeking to end child marriage in the eastern Indian state of Bihar, where nearly two-thirds of girls in some of its rural areas are married before the legal age of 18.

The app, Bandhan Tod, was developed by Gender Alliance - a collective of more than 270 charities in Bihar focused on gender rights - and launched this week by Deputy Chief Minister Sushil Kumar Modi. It is backed by the United Nations Population Fund.

India ranks among countries with the highest rates of child marriage in the world, accounting for a third of the global total of more than 700 million women, according to UNICEF, the United Nations children's agency.

Bandhan Tod - meaning "break the binds" - includes classes on child marriage and dowries, and their ill effects. It also has an SOS button that notifies the team when activated.

"The app is a big part of our efforts to end child marriage in the state," said Prashanti Tiwary, head of Gender Alliance.

"Education is good, but when a young girl wants help because she is being forced to marry before the legal age, the app can be her way out," she told the Thomson Reuters Foundation.

Despite a law banning girls from marrying before they turn 18, the practice is deeply rooted in tradition and widely accepted in Indian society. It is rarely reported as a crime and officials are often reluctant to prosecute offenders.

While boys also marry before the legal age of 21, girls are disproportionately affected.

Early marriage makes it more likely that girls will drop out of school, and campaigners say it also increases risks of sexual violence, domestic abuse and death in childbirth.

Legal efforts have failed to break the stranglehold of tradition and culture that continues to support child marriage, charity ActionAid India said in a report this year.

When the SOS on Bandhan Tod is activated, the nearest small NGO will attempt to resolve the issue. If the family resists, then the police will be notified, said Tiwary.

A similar app in West Bengal state to report child marriage and trafficking of women and children has helped prevent several such instances, according to Child in Need Institute, which launched the app in 2015.

Other efforts include a cash incentive, where the state transfers a sum of money to the girl's bank account if she remains in school and unwed at age 18.

Suppliers of wedding tents in Rajasthan state have stopped dozens of child marriages by alerting officials.

"It will take a change in mindset and behaviour to end child marriage," said Tiwary, who is lobbying the government to raise the marriage age for women to 21, so they have the same opportunities as men.

"But technology provides a practical and accessible way to help prevent it on the ground," she said.