The Pentagon has announced it will investigate a nude photo scandal affecting the armed forces after it was discovered that images of female Marines were being shared online.

Defence Secretary James Mattis condemned those involved in sharing the images and said "all appropriate action" was being taken across service branches.

"Lack of respect for the dignity and humanity of fellow members of the Department of Defence is unacceptable and counter to unit cohesion," he said.

The scandal broke when it was discovered nude photos of female servicewomen were being shared online by male colleagues, often accompanied by obscene, misogynist commentary, in a closed Facebook group called "Marines United."

The group reportedly had upwards of 30,000 members before being shut down, however it has since emerged the scandal runs much deeper.

More photos were uncovered on the military page of photo sharing site Anon-IB, enveloping the entire US armed forces in the scandal.

Some of the photos are believed to have been taken without consent, while others may have been consensual but posted without permission.

The Naval Criminal Investigative Service announced it will investigate the practice, however, according to a top Marine official, fewer than 10 female Marines have made a formal complaint about the images.

General Robert Neller made an impassioned plea for women who may have been affected to come forward with their complaints.

"I'm going to ask them to trust us and I understand why that might be a bit of a reach for them right now," he said.

"The only way that there is going to be accountability in this is if somebody comes forward and tells us what happened to them."

He also vowed to hold those involved accountable and condemned the practice saying "there is no honour in denigrating a fellow Marine."

"If you're participating in this type of behaviour in any way shape or form - you're not helping me or your Marine Corps," he said.

"You know we claim that being a Marine is a special title and something that you earn. There's honour here. But there is no honour in denigrating a fellow Marine in any way shape or form."

The Senate Armed Services Committee will hold a hearing on the matter next week.