Former Democratic presidential candidate and NATO commander Wesley Clark has called for the return of World War II-style internment camps for "disloyal Americans" who support the Islamic State as part of an intensified campaign against Islamic extremism.

The suggestion was a surprise, with pundits saying it was "out of character" for the retired general to make such a call based on his earlier stances against the Bush administration's excessive response to the Sept. 11 attacks.

Clark said the internment camps should be revived, following the mass shooting in Chatanooga, Tennessee.

He recalled that during World War II, "if someone supported Nazi Germany at the expense of the United States, we didn't say that was freedom of speech, we put him in a camp, they were prisoners of war."

He underlined the potential role internment camps could have in battling Muslim extremism.

"If these people are radicalised and they don't support the United States and they are disloyal to the United States as a matter of principle, fine. It's their right and it's our right and obligation to segregate them from the normal community for the duration of the conflict," he said.

Observers said the remarks were "shockingly out of character for Clark" as he became known in progressive political circles after his stint as supreme allied commander of NATO.

Since his critique of the way the Bush administration responded to the 9/11 terror attacks during his presidential campaign, Clark has become a critic of measures violating the Geneva Convention. In particular, he said back in 2006 that policies like torture violate "the very values that [we] espouse."

Earlier this year, Clark also cited the possible threat posed by foreign fighters returning from armed conflicts overseas, adding that he opposed "the politics of fear" eroding democratic institutions and norms.

He also criticised Bush-era neoconservatives who committed excesses under the banner of fighting against terrorism.

But on Friday, he was advocating the revival of a policy widely considered to be among the most shameful chapters in American history: World War II domestic internment camps. Aside from the inherent problems in criminalising people for their beliefs, Clark's proposal seemed based on targeting for state scrutiny those who are not even "radicalised" but who the government thinks may be subject to future radicalisation.

"We have got to identify the people who are most likely to be radicalised. We've got to cut this off at the beginning," Clark said. "I do think on a national policy level we need to look at what self-radicalisation means because we are at war with this group of terrorists."

He also said "not only the United States but our allied nations like Britain, Germany and France are going to have to look at their domestic law procedures."

The Intercept commented: "That radicalisation itself is a highly amorphous and politically malleable concept only makes this proposal more troubling."

Nathan Robinson, writing for America Aljazeera, said that "one might inform him (Clark) that the United States' heinous civil liberties abuses during World War II are often considered a particularly dark patch in the nation's history. The rounding up of more than 100,000 Japanese-Americans — and their placement in squalid camps — was a racist disgrace that the country apologised for in 1988 and left traumatic scars that last to this day. The lesson supposedly learned was that the humiliation and segregation of an entire ethnic group is an indefensible assault on principles of dignity and equality."

"Clark, however, appears to have taken this cautionary tale as a useful suggestion."