US Defense Secretary Ashton Carter on Monday encouraged women troops to participate in groups inspired by "Lean In," the best-selling female leadership and empowerment book written by Facebook executive Sheryl Sandberg.

At a time when the US military is considering opening all combat roles to women, Carter said it is vital the armed services actively look for more women to fill leadership positions.

"The people are the strength of our military, and frankly for the all-volunteer force to stay the strongest ... we need to draw the top talent from the largest and most complete pool of Americans," Carter said at Pentagon appearance alongside Sandberg.

"The simple fact is that women make up half of the population."

The pair spoke after attending a "Lean In Circle" with about a dozen women of different stripes from across the military.

These groups provide a venue for women to find mentors and discuss issues arising from a male-dominated workplace. Only about 15 percent of the US military is female.

"These Lean In Circles have empowered me, have encouraged me to speak up, and speak up for those that may not have the courage," Air Force Major Melody Mitchell told AFP.

"In the military, you are always worried about what your chain of command thinks ... When you break away from that, you can achieve a lot."

Sandberg, 46, is Facebook's chief operating officer. Her 2013 book "Lean In" highlighted the relative scarcity of women in leadership positions, unconscious biases and the reluctance some women encounter in advocating for themselves.

- Nine percent of generals are women -

One famous example she describes is how women tend to apply for jobs only when they are sure they're well qualified, whereas men are more likely to try their luck even if they're only partially matched to a position's requirements.

Sandberg noted that only nine percent of America's generals are women. While she commended Carter and the Pentagon for addressing the problem, she pointed to an ongoing "leadership gap" in the military and in the corporate world.

"It is not clear that we are on a path to fix this; we have a lot more to do," she said. "There are cultural barriers because our expectation of leadership is so overwhelmingly male."

The Pentagon has approved service members to hold Lean In Circles during work hours, and they are encouraged to do so either online or in a military facility.

"This is mission critical if we are going to have the force that we need to defend the United States and our values around the world," Sandberg added.

Last month, two women became the first females to graduate from the Army's grueling Ranger School, though it is uncertain they will end up in front-line fighting roles.

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.

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

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

Carter will have the last word on which -- if any -- positions will remain closed to women.