An Amnesty International report has claimed that North Wales Police may not be aware of the extent of sex trafficking in their area.
The report aimed to provide a 'snapshot' of sex trafficking
It said their monitoring of the problem might not be as good as that found in the Gwent and South Wales forces.
North Wales Police said it had played a comprehensive and active role in protecting victims of the sex trade.
The force said it was "disappointing" Amnesty International had not properly researched the local situation.
The report called Under the Covers, discusses the difficulties police face in identifying sex trafficking victims, and gives examples of unverified cases in north Wales.
It also recommends that a refuge for trafficked women is set up in Wales.
The human rights charity said it was compiled to provide a "snapshot" of the extent of the issue in Wales.
It researched crime records and interviewed police outreach workers and professionals with knowledge of the sex industry to estimate the extent of the problem in Wales.
The report estimated that there are around 60 women at any one time in Cardiff who have been trafficked for the sex trade.
It also found smaller but "significant" numbers of trafficked women in Swansea and Newport.
Evidence of small numbers of trafficked women in small communities in north Wales were uncovered too.
The report's authors researched the work of police in north Wales during Operation Pentameter, a national crackdown on human trafficking in 2006.
SUMMARY OF FINDINGS
In the previous two years, 18 women from overseas had been removed from brothels in Wales and 12 of these had been confirmed as victims of trafficking
Most of the women found had been from the Far East, aged between 18 and 40
Six people had been convicted of trafficking for sexual exploitation in Wales since 2004
Since March 2003, five women from overseas have been referred to a project in London
Source: Under The Covers, Trafficking For Sexual Exploitation in Wales, Amnesty International
As part of the operation, more than 40 police officers searched a number of properties in Rhuddlan, Penmaenmawr, Cemaes Bay, Llanrhuddlad and Colwyn Bay.
Six people were arrested but released pending further enquiries.
But three 18-year-old women from China, Malaysia and Namibia who were found to be working as prostitutes by North Wales Police, were released from covert police premises after 10 hours.
They refused services and told police they believed they had not been trafficked.
Amnesty said it was not known where they went or whether they were still working as prostitutes.
The women had not been arrested and were offered food, clothes, translators and health checks.
The Amnesty report found that while police had been trying to help the women, it pointed out that victims of trafficking were hard to identify because of threats made to them and their families by traffickers.
"Moreover, victims often originate from countries where the police are corrupt and cannot be trusted," the report said.
"These fears greatly influence the behaviour and statements of victims, even after they have been removed from traffickers' control."
The report said the case involving the three 18-year-olds suggested that improved training, a clearer understanding of victim identification and the implementation of national guidelines could help ensure that victims received support.
Chief Supt Chris Corcoran of North Wales Police said: "It is disappointing that Amnesty International has not properly researched the local situation nor approached the force before making its criticisms.
"North Wales Police has played a comprehensive and active role in protecting victims of the sex trade and dealing with those who prey on them."
An Amnesty International spokeswoman said the charity had been in consultation with North Wales Police, along with other forces across Wales.
She said: "We do feel that all police forces in Wales would benefit from enhanced guidance on this matter and we also know that the police are happy to support our call to ensure that there is better support for victims."