US President Donald Trump said Monday he would sign an executive order ridding the US military of what he called "transgender ideology" in a potentially major setback for LGBTQ rights.

"To ensure that we have the most lethal fighting force in the world, we will get transgender ideology the hell out of our military," Trump told a Republican congressional retreat in Miami, listing it as one of "four new executive orders" he will sign related to the armed forces.

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 serving in the armed forces 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 1 July 2017.

But the first Trump administration postponed that date to 2018 before deciding to reverse the policy entirely, sparking criticism from rights groups.

Trump claimed that transgender service members were disruptive, expensive and eroded military readiness and camaraderie among troops.

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 executive order restored the policy set by Obama, but Trump vowed after his election to a second term last year that he would "get transgender out of the military," among other rollbacks to trans rights.

While the number of transgender troops in the American military is fairly small — with estimates of some 15,000 out of more than two million uniformed service members — their dismissal would reduce US forces at a time when the country is already facing difficulties recruiting new personnel.

Biden's outgoing defense secretary, Lloyd Austin, appeared to criticize Trump's plans during a farewell address earlier this 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.