HIGH demand for virgins from mostly Asian clients is fuelling the flow of underage girls into Cambodia's sex trade, the International Organisation for Migration (IOM) says.

About 38 per cent of the women and girls surveyed working in the sex trade in Cambodia entered the industry by selling their virginity, IOM found, while 85 per cent of the clients paying to sleep with them were Asian men.

"The commercial sale of virginity is one of the major routes into commercial sexual exploitation,'' the group said in a report released earlier this week.

"This trade clearly shapes the patterns of trafficking within Cambodia ... it is arguably the largest factor contributing to entry into commercial sex.''

Non-Asian foreigners accounted for 9 per cent of customers paying for sex with virgins, IOM said, while the other 6 per cent was unknown.

More than half the Asian clients were Cambodian men, with the other half made up of Chinese, Taiwanese, South Korean, Japanese, Thai and Filipino men.

Most women were 16- or 17-years-old when they sold their virginity, and the majority said they entered prostitution willingly.

But others told IOM they were tricked by family and friends.

"There was a high use of force and incapacitation through drugs or alcohol at the time of commercial sex,'' the humanitarian group said.

People were increasingly picking up virgins in high-end karaoke bars, massage parlors and other entertainment establishments in the country's urban centres, IOM said, whereas before the trade was found mostly in brothels.

IOM surveyed 203 women and girls working in the sex trade in the tourist hub of Siam Reap, the seaside town Sihanoukville and Koh Kong province, which borders Thailand.

Cambodia has struggled to shed its reputation as a haven for paedophiles, putting dozens of foreigners in jail for child sex crimes or deporting them to face trial in their home countries since 2003.