Three retired Army generals and an advocacy group on Wednesday urged Defense Secretary Jim Mattis against what they say could be a return to "Don't Ask, Don't Tell" for transgender troops.

The military is set to begin accepting transgender recruits under a policy change last year but some of the services still have concerns and Mattis may be set to weigh in with a final decision on fully carrying out the plan, anonymous sources recently told Military Times.

The female officers, who wrote an open letter released by the Palm Center, said Mattis should uphold the existing policy due to evidence that allowing transgender troops will have little negative effect on the services.

"Defense Secretary Jim Mattis has stated that he will make personnel decisions based on evidence about what best promotes force readiness," they wrote. "If he is serious about that commitment, he will maintain existing policy and make clear that there will be no return to the days of forcing capable applicants to lie in order to serve their country."

The retired officers are Lt. Gen. Claudia Kennedy, who served as deputy chief of staff for intelligence during the Clinton administration; Maj. Gen. Gale Pollock, who was deputy surgeon general for force management until she retired in 2008; and Brig. Gen. Clara Adams-Ender, former chief of the Army Nurse Corps.

During his confirmation hearing in January, Mattis said he had no plans to roll back the "Don't Ask, Don't Tell" policy for gay and lesbian service members, which was repealed in 2011, and indicated he would review transgender rules only if those under him at the Pentagon brought concerns.

The Palm Center, which researches and advocates for sexual minorities in the military, said Wednesday it has tracked about two dozen transgender troops who have served openly in the military over the past year and that the new policy appears to be a success.

But the military services, particularly the Marine Corps, have been less enthusiastic about changes such as repealing "Don't Ask, Don't Tell," moving women into combat roles, and integrating transgender troops.

Either way, the issue of transgender service affects a tiny fraction of the total military force.

The Rand Corp. found last year that about 1,320 to 6,630 transgender troops serve in the active-duty forces.

There has been debate over whether the military would have to pay for gender-reassignment surgery as part of its healthcare coverage and whether such treatment might hamper deployment. Rand found the medical costs would be about $2.4 million to $8.4 million annually, a maximum of 0.13 percent of active-duty healthcare costs.