About 100 members of the US military took part in some form of "prohibited extremist activity" over the past year, the Pentagon said Monday as it released new guidelines for service members.

Pentagon chief Lloyd Austin ordered a review in February 2021 of the Defense Department's policies on countering extremism within the ranks.

The review came after the revelation that dozens of former members of the US military took part in the January 6 attack on the US Capitol by supporters of former president Donald Trump.

"The overwhelming majority of the men and women of the Department of Defense serve this country with honor and integrity," Austin said in a statement accompanying the release of the working group report on countering extremist activity.

"They respect the oath they took to support and defend the Constitution of the United States," Austin said. "We believe only a very few violate this oath by participating in extremist activities."

Pentagon spokesman John Kirby said the review found that "about 100" active duty or reserve members of the US military had participated in prohibited extremist activities over the past year.

He declined to specify what type of activity they had been engaged in, but cited advocating the overthrow of the government or "domestic terrorism" as examples of prohibited activities.

The working group does not list specific extremist groups in its new guidelines.

Among its recommendations were increased training and education for service members on what constitutes prohibited extremist activity.

"That includes very specifically, the guidelines for social media, what's permissible and what's not, with respect to extremist prohibited activities," Kirby said.