The February 7 memo became public on Monday as part of a court filing in a case challenging President Donald Trump's late January executive order that was aimed at barring military service by transgender personnel.

"Effective immediately, all new accessions for individuals with a history of gender dysphoria are paused," Hegseth's memo said.

Additionally, "all unscheduled, scheduled or planned medical procedures associated with affirming or facilitating a gender transition for service members are paused," the memo said.

It did not specify what will happen to transgender personnel who are already in the military, but quoted Trump's executive order, which said: "Expressing a false 'gender identity' divergent from an individual's sex cannot satisfy the rigorous standards necessary for military service."

Transgender Americans have faced a roller coaster of changing policies on military service in recent years, with Democratic administrations seeking to permit them to serve openly while Trump has repeatedly sought to keep them out of the ranks.

The US military lifted a ban on transgender troops in 2016, during Democrat Barack Obama's second term as president.

Under that policy, trans troops already serving were permitted to do so openly, and transgender recruits were set to start being accepted by July 1, 2017.

But the first Trump administration postponed that date to 2018 before deciding to reverse the policy entirely.

Trump's controversial restrictions on transgender military service -- which underwent changes in response to various court challenges -- eventually came into force in April 2019 following a protracted legal battle that went all the way to the nation's top court.

Trump's Democratic successor Joe Biden moved to reverse the restrictions just days after he took office in 2021, saying all Americans qualified to serve should be able to do so.

Biden's defense secretary Lloyd Austin appeared to criticize Trump's plans for future restrictions during a farewell address last month, saying: "Any military that turns away qualified patriots who are eager to serve is just making itself smaller and weaker."

Transgender issues have roiled US politics in recent years, as states controlled by Democrats and Republicans have moved in opposite directions on policies ranging from medical treatment to what books on the topic are allowed in public or school libraries.