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US defence minister James Mattis delivers a speech during a press conference at the Nato Defense Council meeting at the NATO Headquarters in Brussels on June 29, 2017. (JOHN THYS/AFP/Getty Images)

The Pentagon approved a six-month delay to lifting the ban on transgender enlistments in the armed services Friday, the last day of June which marks the end of Pride month. Defense Secretary James Mattis cited further review on the impact on "readiness and lethality" in a memo that announced the delay.

Former Defense Secretary Ash Carter had given a July 1 deadline for the military to begin enlisting transgender service members last year. The delay has pushed back that deadline to Jan. 1, 2018. Mattis said in the memo that the subject would be revisited by Dec. 1.

"After consulting with the service chiefs and secretaries, I have determined that it is necessary to defer the start of accessions for six months," Mattis said in a memo that was sent Friday. "We will use this additional time to evaluate more carefully the impact of such accessions on readiness and lethality."

Accession refers to the future enlistment of transgender members to the armed services but doesn't affect the estimated over 15,000 transgender people already serving in the military.

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The announcement comes as a setback to LGBTQ Military advocate group OutServe-SLDN who said in a statement released Friday that they were "disappointed" by the decision and called the delay a "disservice to the transgender community."

"We are disappointed that the Department of Defense has chosen to delay, by six months, implementation of the recruitment policy, thus denying qualified transgender individuals the opportunity to enlist, attend ROTC or enroll in one of the military academies," the group wrote.

"This delay is a disservice to the transgender community and to our military as a whole," the statement continued.

Human Rights Campaign spokesman and Marine veteran Stephen Peters said that the delay "restricts the armed forces' ability to recruit the best and the brightest, regardless of gender identity" and Aaron Belkin, Director of the Palm Center, said that the delay "makes no sense."

"That makes no sense because, as predicted by all of the research, transgender military service has been a success," he said, according to a report in the Military Times.

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Prior to Mattis' announcement, 16 Democrats, organized by Rep. Jared Huffman (D-Calif.) issued a statement to the Defense Secretary encouraging him to deny the request for a delay.

"We strongly encourage you to deny the request for a six-month delay in transgender policy implementation," the lawmakers wrote in a Friday letter to Mattis, according to The Hill. "There are thousands of transgender individuals in our military today. There should be no further delay in implementing this policy and allowing transgender individuals to serve the country they love."

The Trump administration has been under fire for not recognizing Pride month, which was first recognized by President Clinton in 1999 to commemorate the 30th anniversary of the Stonewall Riots. This recognition was put on hold during the Bush administration, then taken back up under Obama.