WASHINGTON (AP) — Defense Secretary Ash Carter has gotten pushback from senior military leaders on whether the Pentagon should lift its ban on transgender people serving in the armed forces, according to U.S. officials familiar with the discussions.

Carter initially told troops in Afghanistan that he was open-minded when asked if the Defense Department was planning to remove one of the last gender- or sexuality-based barriers to military service. But defense officials said members of his top brass told Carter that they had serious reservations.

The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

Military officials are reluctant to publicly discuss their opposition, but much of it centers on questions about where transgender troops would be housed, what berthing they would have on ships, which bathrooms they would use, and whether their presence would affect the ability of small units to work well together.

There also are questions about whether the military would conduct or pay for the medical treatment and costs associated with any gender transition, as well as which physical training standards they would be required to meet.

The military has dealt with many similar questions as it integrated the ranks by race, gender and sexual orientation. And in many cases they raised comparable worries about what effect the change would have on the force, including whether it would hinder small units that often have to work together in remote, confined locations for long periods of time.

Transgender people — those who believe their gender identity is different than the one they were born with and sometimes take hormone treatments or have surgery to become their chosen gender — are banned from military service. But studies and other surveys estimate 15,000 transgender people serve in the active duty military and the reserves, often in secret but in many cases with the knowledge of their unit commander or peers.

Carter, who became Pentagon chief just five weeks ago, told troops in Afghanistan last month that the key question should be "are they going to be excellent service members? And I don't think anything but their suitability for service should preclude them."

What he didn't know at the time was that one of the troops in attendance was a transgender individual who is serving with the full knowledge of that person's commander.

People familiar with the event would not identify the transgender service member or say if that person met or had a photograph taken with the secretary, saying it could put the person's job in jeopardy.

That transgender service member lives in barracks for that person's chosen gender identity, not the one listed on the troop's identification card, said Allyson Robinson, policy director for an association of lesbian, gay, bisexual and transgender military personnel called Service members, Partners, and Allies for Respect and Tolerance for All, or SPARTA. Robinson said the person is "acknowledged as one of the top performers in the unit," and is known to be a transgender individual by others in the unit.

The transgender issue came to the fore as the military struggled with how to deal with convicted national security leaker Chelsea Manning's request for hormone therapy and other treatment for her gender dysphoria while she's in prison. Manning, arrested as Bradley Manning, is the first transgender military prisoner to request such treatment, and the Army recently approved the hormone therapy, under pressure from a lawsuit.

Manning, like other service members discovered to be transgender, would have been discharged, but she first has to finish serving her 35-year sentence at the military prison at Fort Leavenworth, Kansas.

While there are no separate, formal Defense Department studies on the transgender question, there is an ongoing review that looks at the broader issue of Defense Department standards for enlistment, which includes a 40-page list of medical conditions that preclude service.

Recruits must be free of any contagious diseases or medical, physical, mental or psychological conditions that would limit the person's ability to perform, to serve in various places and environments, wear required equipment, and cannot require absences due to needed hospitalization or treatment.

Such conditions include heart problems, cancer, night blindness, sleep apnea, schizophrenia, serious cases of hemorrhoids and eating disorders. It also refers in several places to sexual conditions or disorders, including transgender.

That review, to be completed next year, could provide a mechanism for changing the ban, U.S. officials said.

While the Defense Department has yet to approve any change in regulations, small teams within the military services are gathering information on the issue. And the Army has announced that decisions to discharge transgender service members will now be made at a higher level than unit commander to ensure consistency.

Advocates for changing the transgender rule point to 2011 when gays and lesbians were first allowed to serve openly and military leaders predicted a rise in hate crimes and harm to unit cohesion and readiness. But officials across the services say none of that has happened.

"There were no signs of problems with unit cohesion," said David Stacy, government affairs director for Human Rights Campaign. "And, we don't think this is different in any way."

Robinson, of Service members, Partners, and Allies for Respect and Tolerance for All, acknowledged the issue raises challenging questions for the military. But she said other nations, including Australia, Canada and Britain, have found solutions.

Robinson began her military career as a man, enlisting in the Army at 17, attending West Point and going on to command Patriot missile units in the Middle East and Europe.

After leaving the military in 1999, Robinson became a woman, but said she knew "long before I ever swore my oath or put on my boots that I was different. And I knew that the difference, if people were aware of it, would prevent me from following in my dad's boot-steps, from living out my dream and paying back my country for the freedoms that I enjoy every day."

"All the hard questions have already been answered," she said. "Unless our leaders commit to a real program of answering these questions, then Americans have no way of knowing if what's behind this is truly insurmountable challenges or bias."

The federal government has also moved to expand legal protections for transgender individuals. Last July, President Barack Obama ordered employment protection for gay and transgender employees who work for the U.S. government or for companies holding federal contracts.

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