Defense Secretary Jim Mattis is reviewing concerns from some military service chiefs over the recruitment of transgender troops, the Joint Chiefs chairman said Monday.

Gen. Joe Dunford said the military has no plans to reverse a new policy allowing current transgender troops to serve openly, but is entering the next phase of integration by looking at how they will join the military.

The Pentagon under Defense Secretary Ash Carter and the Obama administration changed policy a year ago allowing the open service and set a July 1 deadline for each branch of the military to come with policies to begin the recruitment.

"There have been some issues raised with regard to challenges of accession [of] transgender individuals and that is what the secretary is reviewing," said Dunford, during an appearance at the National Press Club in Washington. "Let's be clear. Transgender personnel are serving right now and there is no review ongoing that would affect the ability of those currently serving to continue serving providing they can meet the physical and mental qualifications of service."

During his confirmation in January, Mattis said he had no plans to change current policy unless concerns were brought to his desk. Service branches raised concerns in the past about open transgender service and women serving in combat roles, but both integration programs moved forward anyway.

Still, some transgender rights advocates are concerned the Pentagon is now angling to roll back recent advances for those service members, who had served in secrecy before. This month, three retired female generals wrote a letter published by the Palm Center that urged Mattis not to "return to the days of forcing capable applicants to lie in order to serve their country."

About 1,320 to 6,630 transgender troops serve in the active-duty forces, according to the Rand Corp.

Debate has often focused on whether the military would be required to pay for gender reassignment surgeries as part of its health coverage and whether troops who undergo such procedures would be ready to deploy and fight.

Rand found that the medical treatment could cost the military about $2.4 million to $8.4 million annually.