BANGKOK (AFP) - Five Thai policemen and a human trafficking official are under investigation for allegedly receiving free services from a Bangkok massage parlour accused of coercing underage girls into the sex trade, authorities said Monday (Jan 15).

Acting on a tip-off, police raided the 'Victoria Secret' parlour last week in a Bangkok neighbourhood notorious for its gaudy and cavernous brothels where sex is illegal - but openly sold.

They found more than 100 sex workers from Thailand, Myanmar, Laos and China, at least three of whom were under 18 and had been forced into the industry.

A ledger discovered during the raid and circulated by local media named more than 20 officials who had received free food, booze and even massages, with a superintendent among five local cops on the list.

An unnamed human trafficking official and a tax officer were also named among the recipients facing a probe by the anti-graft office.

Monthly receipts dated from March 2014 to October 2017 gave a detailed breakdown of each gratuity.

Interior Ministry official Kongrob Kratumnat confirmed the documents were "real", adding detectives were also examining a computer seized at the parlour.

"There is still a lot of information related to many government officials left on the computer which will take time to investigate," he told reporters.

Reports suggested the brothel was also linked to cross-border trafficking of women from poor South-east Asian countries through Thailand and on to Malaysia.

Thailand is renowned for its laissez-faire attitude towards prostitution, which is illegal but widely tolerated.

Public officials are routinely implicated in kickback scandals allowing the sex industry to flourish, but police rarely face prosecution in a country where wealth and rank shields wrongdoers from the law.

The age and background of another 25 sex workers from the parlour is being checked in case they are also minors, an official from Thailand's Department of Special Investigation (DSI) told reporters on Monday.

"Eighty-five other women could be charged for illegally selling sex and deported," the official added, requesting anonymity.

In 2017 Thailand, which draws millions of migrant workers from poorer surrounding nations, remained on Tier 2 of a US watchlist for failing to meet "the minimum standards" to combat human trafficking.