Girls under the age of 18 are selling sex on Craigslist, according to police in North Vancouver, and are likely forced into it after falling under the influence of a sex-trafficking ring.

North Vancouver RCMP are reporting a spike in underage sex trade workers on the North Shore, some as young as 13, who claim to be 18 or older in their online ads.

Benjamin Perrin, a University of British Columbia professor who is an expert on human trafficking for the sex trade, said police have identified a dozen victims so far in North Vancouver alone.

"Police believe there are many more victims, many of them are under the control of what we're being told is a sex- and drug-trafficking ring," Perrin said Tuesday on CTV's Canada AM.

According to North Vancouver RCMP Const. Shannon Kitchen, some girls, but not all, are working alone and not under the control of a pimp.

"In some cases we've been told about initially a relationship that's been considered maybe a boyfriend-girlfriend relationship. And then that relationship's turned into, 'now you're working for me, you owe me money,'" Kitchen told CTV Vancouver.

Many of the ads are explicit, Perrin said, and put the girls in a vulnerable position as they promise unprotected sex.

"These young girls are being exposed to very serious health risks," Perrin said.

He also points out that while some of the girls come from troubled homes and communities, which makes them particularly vulnerable to sex traffickers, others hail from stable families and well-off neighbourhoods.

"So parents really need to look out for warning signs too, like new friendships with people who are not just male but also possibly female, giving large gifts, (having) a new cellphone that's always being answered at any time of the day," Perrin said.

He also said that the problem is not relegated to young girls in North Vancouver. Teenagers across Canada are using online tools to sell sex, he said.

Perrin said young women are often recruited through social networking websites such as Facebook, which offer ample opportunities for sex traffickers to prey on vulnerable girls.

"We know that sex traffickers have used Facebook as well as a tactic to identify vulnerable individuals and once they're part of a school network, Facebook does the work for them," he said.

Perrin said he has also met with officials from both the RCMP and Craigslist to express concern about how easy it is for underage girls to pose as adults.

Craigslist has shut down its erotic services section in the U.S. and now has an adult section that requires users to identify themselves. This is not available yet on the Canadian site.

"We're really hoping Craigslist will step up," Perrin said.