"This new policy will enable the military to apply well-established mental and physical health standards – including those regarding the use of medical drugs – equally to all individuals who want to join and fight for the best military force the world has ever seen," White House Press Secretary Sarah Huckabee Sanders said.

The new policy was promptly assailed by congressional Democrats and civil rights groups. House Democratic leader Nancy Pelosi tweeted, "No one with the strength & bravery to serve in the US military should be turned away because of who they are. This hateful ban is purpose-built to humiliate our brave transgender members of the military who serve with honor & dignity."

The Human Rights Campaign, the nation's largest LGBT civil rights organisation, accused the Trump administration of pushing "anti-transgender prejudices onto the military".

"There is simply no way to spin it, the Trump-Pence Administration is going all in on its discriminatory, unconstitutional and despicable ban on transgender troops," said HRC president Chad Griffin.

The Trump-Pence Administration is going all in on its discriminatory, unconstitutional and despicable ban on transgender troops.

Chad Griffin, Human Rights Campaign

Trump received recommendations from Defence Secretary Jim Mattis in February for dealing with transgender individuals serving in the military. The White House said Mattis and Homeland Security Secretary Kirstjen Nielsen agreed with the policy.

Earlier Saturday, Pentagon spokesman Major David Eastburn said the announcement of a new policy would have no immediate practical effect on the military because the Pentagon is obliged to continue to recruit and retain transgender people in accordance with current law.

The issue has become mired in a complicated string of political statements, court decisions and policy reviews since Trump first stunned his administration with his tweets last July.

It's unclear how much impact the court decisions will have on Trump's decision. Activist groups had worried the administration could enact such strict enlistment and health care restrictions that it would become all but impossible for transgender troops to join or continue serving.

Under guidelines presented in December, the Pentagon could disqualify potential recruits with gender dysphoria, those with a history of medical treatments associated with gender transition and those who underwent reconstruction. Such recruits could be allowed in if a medical provider certified they've been clinically stable in the preferred sex for 18 months and are free of significant distress or impairment in social, occupational or other important areas.

Transgender individuals receiving hormone therapy must be stable on their medication for 18 months.

The requirements make it challenging for a transgender recruit to pass. But they mirror conditions laid out by President Barack Obama's administration in 2016, when the Pentagon initially lifted its ban on transgender troops serving openly in the military.

AP