WASHINGTON (AFP, REUTERS) - About 100 members of the United States military took part in some form of "prohibited extremist activity" over the past year, the Pentagon said on Monday (Dec 20) as it released new guidelines for service members.

Pentagon chief Lloyd Austin ordered a review in February 2021 of the Defence 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 Jan 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 Defence serve this country with honour and integrity," Mr 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," the former US army general 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.

Among the 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," Mr Kirby said.

The Pentagon however stopped short of creating a list of extremist groups that military members cannot join and declined to say explicitly whether refusing to view President Joe Biden as America’s legitimate leader was a violation of policy.

The new guidelines  includes everything from prohibiting“liking” extremist content on social media to fundraising or demonstrating for an extremist organisation.

Punishment, if there is any, could be up to local commanders.

Still, the Pentagon stopped short of prohibiting membership in any group, from the Proud Boys to the Oath Keepers and the Ku Klux Klan.

"If we got into coming up with a list of extremist groups, it would be only probably as good as the day we published it because these groups change,” Mr Kirby said.

US defence officials said, however, that the guidelines announced on Monday meant that meaningful, active participation in any such group would be impossible.

Mr Adam Smith, the top Democrat on the military oversight committee in the House of Representatives, praised the updated policy.

“Protecting our service members from the threat of violent extremism is far too important for partisan misinformation,” he said in a tweet.