Faint tunes drift out from an empty pub as darkness fills the misty valley of downtown Mae Hong Son.

A handful of local diners and the odd souvenir shop light up its main street, but this seems to be about as far as nightlife gets in this remote northern Thai province.

Recent events have cast it in another light: An ongoing police investigation has uncovered a prostitution ring involving minors, patronised by government officials and even allegedly run by a police senior sergeant major over a span of at least two years.

Three young women say they were coerced into the trade. Mae Hong Son's governor has had to fend off allegations that he bought sex from a minor, while a local politician from Nonthaburi province north-west of Bangkok confessed to the crime. Nine policemen have been sacked so far, including three accused of gang-raping a minor.

Perhaps most damning is the allegation that senior government officials visiting Mae Hong Son are routinely feted with food and drink, and offered sex with girls.

As the probe widens, human rights activists fear there will be attempts to hide evidence of official complicity.

The scandal has riveted a nation, and turned the small border province, once touted as Thailand's "happiest" province and better known for its rugged mountains and ethnic minorities, into an unwitting icon of child sex tourism.

"Our students get teased about this when they travel outside the province," Mr Chartchay Noisakul, who heads the education division in Mae Hong Son municipality, told The Sunday Times. "They have to hide the fact that they are from Mae Hong Son."

Three alleged victims have been moved out of the province under a witness protection programme. According to those close to the investigation, one was just 14 when she was beaten up and forced to sell sex.

Mae Hong Son-based social worker Thiphawan Kamonthammachot, who has spoken to her, said the girl had to work whenever she got a call from her pimp.

"Even when she was having a meal with her mother, she had to leave," said Ms Thiphawan. "If she didn't, the pimp would send people to find her at home." She was made to have sex with men as often as 10 times a week, and at one point, even with a teacher in her school. For each session, she was paid 1,000 baht (S$40.50).

Non-governmental organisations and law enforcement officials have long observed that certain regions in northern Thailand are vulnerable to child prostitution. A report last year on the sexual exploitation of children in tourism, published by Bangkok- based non-profit organisation Ecpat International, noted: "Children from ethnic minorities, such as those from hill tribe communities in northern Thailand, are especially vulnerable."

The majority of those buying sex from children in Thailand are its nationals, the report added.

Thai police's Anti-Trafficking in Persons Division data shows that the number of sex trafficking cases nationwide has remained constant between 2015 and last year, though its commander Kornchai Klayklueng said the number of child prostitution cases has fallen.

Police cite the northern provinces of Chiang Mai and Chiang Rai as hot spots for sex trafficking and child prostitution, and say the situation in neighbouring Mae Hong Son is not as serious. Still, it remains under close watch.

Locals argue its notoriety is undeserved, but concede that child prostitution takes place. Mr Prasert Pradit, a retired education official and head of Mae Hong Son's civic council, said he was approached by government officials trying to arrange trysts on behalf of their seniors from out of town, when he ran a restaurant some 10 years ago. "Ajarn, mee dek mai? (Teacher, do you have a child?)," they used to whisper to him, much to his disgust.

Unlike the loud, neon-lit establishments typical of Thailand's more infamous red-light districts, the sex trade in Mae Hong Son is conducted discreetly, with prostitutes sent directly to hotel rooms.

It is the product of social conditions, say researchers. Mae Hong Son is a poor and sparsely populated province of about 270,000, which hosts a revolving door of government officials sent by Bangkok.

Many of its ethnic minority households near the border with Myanmar struggle to make a living without full Thai citizenship and the ability to speak Thai fluently. They put their children in dormitories downtown closer to their schools, often with the bare minimum to survive, making them easy targets for pimps.

Teenage pregnancy rates there are among the highest in the nation. According to the Ministry of Public Health, 5.4 per cent of Mae Hong Son's girls aged between 15 and 19 - or 525 girls - gave birth in 2015.

With the unwelcome spotlight on the province, social workers fear the small, tightly knit communities in Mae Hong Son may now turn against the young women who say they were coerced into selling sex. "No one was forced," said Mr Prasert, who thinks this is more a case of minors trying to earn money to keep up with their peers.

Regardless of consent, it is a crime to buy sex from someone under 18 years old in Thailand.

Amid concerns that senior government officials may be spared, Police Major-General Kornchai stressed there will be no cover-up.

The gossip, meanwhile, has devastated families of the alleged child sex-trafficking victims. One of the girls' mothers shut herself in the house and had to be coaxed out by a social worker to seek treatment for a serious illness. "My daughter called me," the woman told social workers last week. "I told her: 'I love you very much. I miss you.' She told me she will work to earn money and asked me to rest."