US President Donald Trump has issued an order banning transgender individuals who require significant medical treatment, surgery or medications from serving in the military.

Transgender servicemen and women currently serving in the military may remain in the ranks, but the Pentagon may require them to serve according to their gender, the White House said on Saturday (AEDT).

The White House said Defence Secretary Jim Mattis had found transgender individuals with a history or diagnosis of “gender dysphoria” presented “considerable risk to military effectiveness”.

“This new policy will enable the military to apply well-established mental and physical health standards … equally to all individuals who want to join and fight for the best military force the world has ever seen,” the White House said.

The memo defined the transgender ban as applying to individuals or “those who may require substantial medical treatment, including through medical drugs or surgery”.

Secretaries of defence and homeland security might exercise their authority to implement any appropriate policies concerning military service by transgender individuals, the memo added.

The Human Rights Campaign, the nation’s largest LGBT civil rights organisation, has denounced Mr Trump’s “hastily tweeted troop ban” that has turned into official policy.

“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,” HRC president Chad Griffin said.

“Donald Trump and Mike Pence are pushing their extreme anti-transgender prejudices onto the military over the overwhelming objections of bipartisan officials, national security leaders and the American public.”

A 2016 study commissioned by the US Defense Department found that allowing transgender people to openly serve in the military would have a “minimal impact” on readiness and health-care costs, largely because there are so few in the military’s 1.3 million-member force.

The decision follows US President Donald Trump announcing transgender individuals would not be allowed to serve in the U.S military “in any capacity” in a series of tweets in July 2017.

After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow……

— Donald J. Trump (@realDonaldTrump) July 26, 2017

….Transgender individuals to serve in any capacity in the U.S. Military. Our military must be focused on decisive and overwhelming…..

— Donald J. Trump (@realDonaldTrump) July 26, 2017

….victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail. Thank you

— Donald J. Trump (@realDonaldTrump) July 26, 2017

Mr Trump’s policy reversed the Pentagon’s 2016 decision to allow transgender individuals to serve.

A number of federal judges have issued rulings blocking Mr Trump’s ban, saying it would probably violate the right, under the US Constitution, to equal protection under the law.

The memorandum said 8980 service members reportedly identify as transgender, with 937 active duty service members diagnosed with “gender dysphoria” since June 30, 2016.

On Friday, the Pentagon reaffirmed that it would continue to comply with federal law.

-with AAP