The international trafficking of women to work as prostitutes is likely a key factor in the spread of HIV across South Asia, according to a study released Wednesday that found high rates of the virus in Nepali girls and woman who worked in Indian brothels.

The study in the Journal of the American Medical Association examined 287 Nepali women and girls who had been trafficked into Indian brothels and returned to Nepal between 1997 and 2005. It found that some 40 percent of them were HIV positive, with the figure rising to 60 percent among those who had been trafficked before the age of 15.

"The high rates of HIV documented herein support concerns that sex trafficking may be a significant factor in the expansion of the South Asian HIV epidemic," said the report on the study run by Jay G. Silverman of the Harvard School of Public Health

South Asia is one of the areas worst hit by HIV and AIDS, with some 3.5 million people infected in India alone. The report estimated that some 150,000 girls and women are trafficked each year across the region.

A U.S. State Department report released last month found that India has the world's largest human trafficking problem.

That report estimated that tens of millions of Indians were subjected to forced labor and said sex trafficking was common. It noted the Indian government had taken some steps to combat sex trafficking.