A plan to allow transgender recruits to join the United States military beginning on Saturday has been delayed for six months by Defense Secretary Jim Mattis, a Pentagon spokeswoman said on Friday.

The 11th-hour decision by Mr. Mattis will allow service leaders to “review their accession plans and provide input” as they consider what impact adding transgender recruits would have on “the readiness and lethality of our forces,” the spokeswoman, Dana W. White, said in a statement.

The delay was announced one year after the Pentagon lifted its ban on transgender people serving openly in the armed forces.

Military leaders formally requested last week that Mr. Mattis approve a delay, The Associated Press reported on Friday. The news agency quoted a memo it obtained in which the defense secretary noted that his decision to defer “does not presuppose the outcome of the review.”