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Defense Secretary Jim Mattis condemned some Marines' conduct as "unacceptable" after reports that members of an online group shared nude photos of female Marines accompanied by sexually degrading comments.

"Lack of respect for the dignity and humanity of fellow members of the Department of Defense is unacceptable and counter to unit cohesion," Mattis said Friday in his first public comments on the growing scandal. The controversy has shaken the U.S. armed services since the first reports emerged almost a week ago.

"We will not excuse or tolerate such behavior if we are to uphold our values and maintain our ability to defeat the enemy on the battlefield," Mattis said.

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The Pentagon chief is expected to meet with military and civilian leaders in the coming days, while Marine officials are set to brief members of Congress on their investigation next week.

Gen. Robert Neller, the commandant of the Marine Corps, told reporters Friday that a task force has been set up to address changing behaviors within the branch. He added that no charges have been filed, and it remains unclear how many people may be involved.

The group "Marines United" began on Facebook and had more than 30,000 followers — potentially a mix of active-duty personnel, veterans and civilian members.

Related: Marines Photo Scandal: What Can Revenge Porn Victims Do?

Neller added that at least 10 women's images were used on the page without their knowledge or consent. Since it was taken down, a second Marines United group was started on Facebook, as first reported by Business Insider, and a similar exchange of pictures was discovered on the image-sharing board Anon-IB.

Neller implored for more female Marines who may be victims to come forward.

"I'm going to ask them to trust us. I understand that that might be a bit of a reach for them right now," he said. "The only way there will be accountability is if someone comes forward."

One female veteran of the Marines who came forward Wednesday at a news conference said a picture of hers was used on the Marines United page without her consent. She told reporters that she was "disheartened and disgusted."