Convicting traffickers operating across state and national borders is the biggest challenge law enforcement agencies face today, an Indian police officer said after winning the US government's 2017 Trafficking in Persons Hero award.

The US State Department named Mahesh Muralidhar Bhagwat one of eight winners this year, citing his leadership in combating modern slavery in India, his role in elevating human trafficking as a government priority and his innovative approach to investigating cases.

Bhagwat, 48, was presented the award in absentia in Washington on Tuesday.

"This award is a recognition for the cause that I have been fighting against for more than a decade," said Bhagwat, now police chief in Rachakonda district of southern Telangana state.

"It has been a long journey from the first trafficking case I stumbled upon sometime in 2004.

"I was shocked that school buses were being used to ferry trafficked girls to resorts on the outskirts of the city for prostitution," he said of his first case. "It rattled me."

Bhagwat, who worked as a civil engineer before joining the police, has been credited with closing 25 brothels in less than a year and participating in one of the country's largest crackdowns on labour trafficking, rescuing more than 350 children forced to work in brick kilns.

He called for better cooperation and protocols to combat trafficking effectively.

"Dealing with traffickers who operate across state and national borders requires a standard protocol that agencies across these boundaries can effectively use," Bhagwat told the Thomson Reuters Foundation.

India has an estimated 40 percent of almost 46 million people enslaved worldwide, according to the 2016 Global Slavery Index.

The 2017 Trafficking In Persons report released on Tuesday recognised India's efforts to identify victims, complete investigations and increase spending on rehabilitation of survivors.

It also calls for the Indian government to increase prosecutions and convictions for all forms of trafficking and fully resource anti-human trafficking units in all districts.