From their boot camp buzz cuts to the "high and tight" style standard for the Marine Corps, a spartan trim is inseparable from their war-fighting discipline, Pentagon Joint Chiefs Chairman General Mark Milley said Tuesday.

Milley was asked whether social distancing practices, which have prevented many worldwide from keeping their barber appointments, should be implemented in the US military, after a video surfaced of about two dozen of Marines lined up for their regular cut at Camp Pendleton in California.

While most were keeping their distance, it was not the recommended six feet, and none had masks on, the video showed.

"Whether they are marines, or soldiers, or airmen, or sailors, you know discipline is a fundamental function of our force," Milley told reporters.

"And for many, many years the United States military has had hair standards.... Yes, I think Marines should get haircuts. "

Milley cited the legendary US Marines' assault on the Japanese-held island of Iwo Jima during World War II in early 1945.

"That Marine victory was the result of incredible discipline," he said.

"It may seem superficial to some, but getting a haircut is part of that discipline."

Defense Secretary Mark Esper, who is already dealing with nearly 600 COVID-19 infections aboard an aircraft carrier and 4,769 cases in total across some 150 military installations, said it was one of myriad issues as they try to both protect the US forces and maintain battle readiness.

"There's no doubt in my mind that you could go to any camp, fort, base, you name it and find somebody not following the guidance," he said.

"That's something I would not have thought of putting into the guidance, the haircut policy," he added.

To which Milley jumped in: "Don't take that as guidance yet. (There are) a lot of ways to do haircuts."