New female recruits to the United States Marines are being trained alongside men for the first time.

A platoon of about 50 women began training at boot camp on Saturday in South Carolina alongside five all-male platoons.

In a statement, the Marine Corps said this marked the first time men and women had ever been integrated in basic training.

"About 50 female recruits, which is smaller than what is typically part of a training cycle, are prepared to begin training at the depot on 5 January," the statement said.

"The programme of instruction remains unchanged and the recruit training order continues to be the standard for training recruits."

The standard Marine Corps boot camp schedule involves a series of punishing physical fitness tests, including running three miles in less than 28 minutes and completing 70 crunches in two minutes.

Recruits are then instructed in Marine Corps history and culture and military protocols, learn martial arts, complete obstacle courses, and trained in marksmanship.

"[Mixed training] enables appropriate acclimation to the training environment, development of relationships with drill instructors, and focus during the transformation of young women and men into United States Marines."

The historic experiment in training men and women side by side would still see recruits being taught by instructors of the same gender, the Corps also said.

Women will live on a separate floor in the barracks and still mostly train within their single-sex platoons, apart from the final endurance test, known as the Crucible, and some other larger-scale exercises.

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After the 13-week boot camp, the Marines will evaluate the training results to determine whether mixed-gender instruction will become the norm.

Women have been able to join the Marines permanently since 1948 but were banned from most frontline combat roles until 2016.

The leader of an organisation which advocates for women in the armed forces said the Marines' move was far from progressive, and in fact long overdue.

Retired army colonel Ellen Haring, the chief executive of the Service Women's Action Network, said the Marines were badly lagging behind the army, navy and air force in the US.

Advocates for women in the military said the move was long overdue (AP)

"The Marine Corps' recent announcement that it would integrate an all-female platoon within a company of all-male platoons on a trial basis comes decades after all of the other services integrated all of their basic training units," she said.

Sources within the Corps told The New York Times the decision to integrate a female platoon with male platoons was in fact taken to maximise efficiency.

Not enough women had joined up for the winter training cycle so it was more practical to include the smaller female group alongside the men rather than run a separate women-only boot camp.

In a statement Col Haring said any separation of men and women during boot camps reflected "systematic inequality" and also violated the constitutional right of "equal protection".

It was no coincidence the Marine Corps had the fewest women of any of the services but saw the highest rates of sexual assault, she also said.

"We encourage the Marine Corps to conduct this experiment transparently, fairly and with independent expert oversight so as to assure sound methodology and reliable results."

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A retired Marine lieutenant colonel who was fired by the Corps after criticising the way women recruits were trained said the experiment should be the beginning of a broader cultural shift.

"This may seem small, but it’s huge," Lt Col Kate Germano told the New York Times. "It will be difficult for the Marines to go backward after this.