Pentagon spokeswoman Dana White said in a statement that Mattis accepted a recommendation to defer transgender applicants until Jan. 1.
The Pentagon ended its ban on openly transgender people serving in the U.S. military in 2016 under the Barack Obama administration. It was expected to start allowing transgender people to begin enlisting this year, provided they had been "stable" in their preferred gender for 18 months.
A delay under President Donald Trump's administration alarmed transgender advocates.
"We are disappointed in this needless delay because the thousands of highly trained and qualified transgender service members openly and proudly serving our nation today have proven that what matters is the ability to accomplish the mission, not their gender identity," Stephen Peters of the Human Rights Campaign said in a statement.
Last year, then-Defense Secretary Ash Carter cited a study by the RAND Corporation saying there were about 2,500 transgender active-duty service members and 1,500 reserve transgender service members.
Rand's figures were within a range, which at the upper end reached 7,000 active duty forces and 4,000 reserves.
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