"It involves people from the highest level of their profession," said Nancy Scheper-Hughes, founding director of Organs Watch, an academic research project at the University of California, Berkeley.

Some surgeons are "willing to collaborate with the lowest levels of society - with criminal networks, brokers and with kidney hunters, who are the absolutely necessary factor," she said on Thursday.

Scheper-Hughes, a professor who is also the director of the university's medical anthropology program, made her remarks at the Vienna Forum to Fight Human Trafficking. Organs Watch has branches in 10 countries with anthropologists, human rights activists and doctors who volunteer, some of them anonymously, she said.

Illegal organ transplants made headlines recently when a man in India was accused of illegally removing hundreds of kidneys, sometimes from poor labourers held at gunpoint.

Indian police have said he headed an illegal organ transplant ring based in the New Delhi suburb of Gurgaon. Authorities believe his group sold up to 500 kidneys to clients who traveled to India from around the world in the past nine years.

"We don't really know how many people are trafficked for organs," Scheper-Hughes said, adding that a conservative estimate for the number of trafficked kidneys was 15,000 each year.