The head of the US Navy on Friday criticized a Marine Corps study that found all-male combat teams performed better than those that included women.

The experiment has caused a stir in the US military because it raises questions about the efficacy of female frontline fighters just as the armed services are preparing to open all combat positions to women next year.

During the three-month trial, a Marine battalion was divided into all-male and "gender-integrated" teams who performed a series of demanding tasks and training exercises in the sweltering California desert.

Experimenters found that the men-only squads performed better on 69 percent of tasks, and women troops were more likely to get injured.

Additionally, all-male rifleman infantry squads had better weapons accuracy compared to the gender-integrated groups.

But speaking on National Public Radio (NPR), Navy Secretary Ray Mabus suggested the study may have been flawed.

"It started out with a fairly large component of the men thinking this is not a good idea, and women will never be able to do this," Mabus said.

"When you start out with that mindset, you're almost presupposing the outcome."

Mabus's comments are particularly significant because in the United States the Marine Corps falls under the authority of the Navy.

"A more diverse force is a stronger force," Mabus said. "If you have the same outlook, if you have the same mindset you don't get much innovation."

Mabus pointed to part of the study that said because women get injured more frequently, Marines would over time lose "combat effectiveness."

"That was not shown in this study," Mabus said. "That was an extrapolation based on injury rates. I'm not sure that's right."

He suggested the Marines could have set a higher standard for women to get into the study in the first place.

"In terms of the overall population, men or women, there were simply no standards," Mabus said.

President Barack Obama's administration decided in 2013 that all combat positions should be open to women by 2016, including the infantry and special forces.

The military services can request exceptions, provided they are justified by operational constraints.

Defense Secretary Ashton Carter will have the last word on which positions will remain closed to women, in January 2016.

Last month, two women became the first females to graduate from the Army's grueling Ranger School, to the delight of female peers hoping to see more frontline roles open up for women combat leaders.