Defense Secretary Jim Mattis is leaving an Obama-era policy on transgender military service members largely intact, saying he needs input from an expert panel to determine the best way to implement President Trump's ban that would keep transgender people from serving in the U.S. military.

Trump barred transgender would-be recruits from signing up, but he gave Mattis discretion to decide the status of transgender people who are already serving.

Mattis says he'll convene the panel from the Department of Defense and the Department of Homeland Security, with the goal of promoting "military readiness, lethality, and unit cohesion" — while also following legal and budgetary constraints.

"In the interim, current policy with respect to currently serving members will remain in place," Mattis said in a statement Tuesday night. "I expect to issue interim guidance to the force concerning the president's direction, including any necessary interim adjustments to procedures, to ensure the continued combat readiness of the force until our final policy on this subject is issued."

The issue directly affects thousands of service members. An estimate by the Rand Corp. has suggested that there could be as many as 6,600 transgender troops on active duty and more than 4,100 in the Reserves. Advocates have said the number is higher — up to 15,000 people.

Trump signed a memo instituting the ban last Friday, setting an effective date of March 23, 2018, to allow leaders at the defense and homeland security departments to implement it. The latter includes the Coast Guard, which is also covered by the policy.

At least two lawsuits are targeting Trump's new ban, with human rights groups representing transgender service members and those who want to serve.

Dating back to last summer, when the Obama administration announced that transgender people could openly serve in the military, the executive branch has made a distinction between those who are already in the service and those who want to enlist.

While Trump has said that the U.S. won't allow transgender people "to serve in any capacity in the U.S. Military," he has maintained a similar divide.

In his update on the Pentagon's plan, Mattis said, "The department will carry out the president's policy direction, in consultation with the Department of Homeland Security."

Mattis continued, "As directed, we will develop a study and implementation plan, which will contain the steps that will promote military readiness, lethality, and unit cohesion, with due regard for budgetary constraints and consistent with applicable law."

Gay and lesbian troops have been able to serve openly in the U.S. military since 2011; transgender service members were allowed to do the same in 2016, through an order from then-Defense Secretary Ash Carter. But their status was thrown into jeopardy after Trump called for a total ban.

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