Online marketplace Craigslist says it needs to look at measures which can prevent the misuse of the site, following allegations that it helps promote prostitution.
But the firm has responded by asking if the crimes had been reported to the police, adding it was "combating trafficking".
"If Craigslist was misused, we want to learn more so we can improve our preventative measures," the BBC quoted the firm as saying.
The letter said that Craigslist was now the choice of traffickers because it was "so well known and there are rarely consequences to using it for these illegal acts".
The firm's chief executive, Jim Buckmaster, replied by asking if the "perpetrators are behind bars" and if the advocacy groups who placed the adverts could "let us know where the police reports were filed".
"We have been unable thus far to identify police reports matching the crimes you describe," the statement read.
"If anyone committing such crimes has not yet been apprehended and prosecuted, we want to do everything in our power to assist the police in making that happen," added the statement.