Thailand is failing to provide treatment to drug users most at risk of AIDS despite its reputation as a pioneer in the global battle against the disease, Human Rights Watch said Thursday.

The Thai government estimates that 40 to 50 percent of injection drug users are living with HIV, a figure that has not changed over the past two decades despite general success in preventing infections, the group said in a report.

"Thailand wants to be seen as a success story in the fight against AIDS, yet it is failing to address the epidemic among the population hit hardest by HIV," said Rebecca Schleifer, an advocate with Human Rights Watch.

"The Thai government has recognised that the HIV infection rate is 'unacceptably high,' and it has the expertise to address this public health emergency," she said.

The report blames the problem on police harassment and discrimination against drug users, saying health care workers deny antiretroviral treatment to people who need it if they are using illicit narcotics.

Many injection drug users were also driven into hiding and afraid of authorities after former premier Thaksin Shinawatra led a "war on drugs" in 2003, when at least 2,500 people died in extrajudicial killings, the group said.

Thailand has generally been lauded for its universal treatment programme, which has included a controversial battle with pharmaceutical companies for cheap or generic versions of cutting-edge treatments.