One site, PSfights.com, hosts videos of men beating women.
Nigel Gwilliam, head of digital at the Institute of Practitioners in Advertising, told Media Week that at least two of the IPA's members are investigating the possibility of legal action against the networks responsible.
The issue emerged after media agency executives were contacted by investigators for a special edition of BBC show Panorama that will highlight problems of cyber-bullying.
Gwilliam said: "Initial results of an investigation have found that in certain cases the terms and conditions (in contracts) between agencies and ad networks have been broken."
He said the IPA had issued an alert to all members asking them to be extra vigilant about where their inventory appears.
"It's unclear whether these breaches of terms and conditions were mistaken or intentional, but the fact that they have been breached is not in doubt," said Gwilliam.
"What the industry needs to do now is make sure self-regulation is robust. The IPA is in dialogue with ISBA and the IAB about the issue."
Meanwhile, IASH - the trade body for online ad networks - denied that its members have been involved in recent allegations, but has warned member networks to tighten up their procedures.
Any networks found to breach IASH's code of conduct will face suspension and possible expulsion.
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