President Donald Trump has announced on Twitter that he will ban transgender people from serving in the US military in any capacity, a reversal of an Obama administration decision that would have officially allowed them to serve.

Citing the need to focus on "victory", Mr Trump said that the military cannot accept the burden of higher medical costs and "disruption" that transgender troops would require.

"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military," Trump wrote on Twitter. "Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail."

The missive comes after Mr Trump's defence secretary Jim Mattis delayed the implementation of a plan to accept transgender troops by six months. The decision came one day before the deadline for the military to update its medical standards to accommodate transgender service members.

The six-month delay was requested by the Joint Chiefs of Staff and would allow a further review of how allowing transgender recruits to serve would affect the military's lethality, Mr Mattis said in a memo. That review was due by early December.

Mr Mattis cautioned at the time that the delay "in no way presupposes the outcome".

"Since becoming the Secretary of Defence, I have emphasised that the Department of Defence must measure each policy decision against one critical standard: will the decision affect the readiness and lethality of the force?" Mr Mattis said last month in a statement. "Put another way, how will the decision affect the ability of America's military to defend the nation? It is against this standard that I provide the following guidance on the way forward in accessing transgender individuals into the military Services."

The Pentagon was not immediately available to comment.

The Senate's most prominent veteran, Armed Forces Committee Chairman John McCain, called Mr Trump's announcement "unclear", an example of why policy announcements should not be made on Twitter, and inappropriate given an ongoing Pentagon study on the issue.

"I do not believe that any new policy decision is appropriate until that study is complete and thoroughly reviewed by the Secretary of Defense, our military leadership, and the Congress," said Mr McCain, a Navy pilot and prisoner of war during the Vietnam War.

The senator noted there were already transgender people serving in the US military: "There is no reason to force service members who are able to fight, train and deploy to leave the military - regardless of their gender identity.

Tony Perkins, president of the Family Research Council advocacy group, was among those praising the announcement. "Our troops shouldn't be forced to endure hours of transgender 'sensitivity' classes and politically correct distractions," he said.

Under the former defence secretary, Mr Carter, the military lifted the ban on transgender troops and was given one year to determine how to implement a policy that would allow transgender service members to receive medical care and would ban the services from involuntarily separating people in the military who came out as transgender.

Thousands of troops currently serving in the military are transgender, and some estimates place the number as high as 11,000 in the reserves and active duty military, according to a RAND Corporation study that has been cited by the Defence Department.

Following Mr Mattis' announcement last month, Aaron Belkin, the director of the Palm Centre, a think tank that has helped the Pentagon research transgender people serving in the military, released a statement condemning the move.

"For the past year, transgender troops have been serving openly and have been widely praised by their Commanders, as is the case in 18 allied militaries around the world including Israel and Britain," Mr Belkin said. "Yet members of Congress are denigrating the value of military service by transgender troops, and Service Chiefs are pressuring Secretary Mattis to continue the transgender enlistment ban despite having no new arguments or data to back up their long-discredited assertions.

"In light of the success of transgender military service, the extensive research confirming that inclusive policy promotes readiness, and the sad history of 'don't ask, don't tell', loyally-serving transgender troops deserve to know whether Secretary Mattis stands by the claim he made at his confirmation hearing that LGBT troops can indeed serve in a 'lethal' military. Stonewalling on full inclusion will, just like 'don't ask, don't tell', compromise military readiness."

Richard Soccarides, a longtime gay rights advocate who advised Bill Clinton on gay men and lesbians serving in uniform, called Mr Trump's decision "a tragic betrayal of all LGBT Americans and a shameful effort to win over social conservatives at the expense the vulnerable".

"But, what Trump will now learn: don't mess with us," Mr Soccarides said. "This gets him a fight he will not win."

Mr Carter also criticised the decision on Wednesday.

"To choose service members on other grounds than military qualifications is social policy and has no place in our military," Mr Carter said in a statement, noting there were already transgender individuals serving "capably and honorably." 

Gay rights and transgender organisations have long threatened legal action against the Pentagon if it decided to roll back or hinder the service of transgender troops. Their legal and political advocacy successfully convinced Congress and President Barack Obama to repeal the Clinton-era "don't ask, don't tell" ban on gay troops through legislation.

Mr Trump's announcement comes two weeks after the House rejected an amendment to the annual defence policy bill that would have blocked the Pentagon from offering gender transition therapies to active duty service members. Twenty-four Republicans joined all 190 Democrats voting to reject the measure.

But the issue has remained a pet cause for House conservatives, who believe the federal government should not be funding gender reassignments. Conservatives have offered several amendments to a pending appropriations bill funding the military that would target transgender service members. But not would exclude them from serving entirely.

During the debate on the initial amendment, Republican Duncan Hunter, a member of the House Armed Services Committee, argued for the therapy restriction but explicitly stopped short of calling for an overall ban: "We're not stopping transgender people from joining. We're saying taxpayers in this country right now are not going to foot the bill for it."

American Civil Liberties Union attorney Joshua Block called Mr Trump's action outrageous and desperate, saying the president rejected the "basic humanity" of transgender service members.

"This has been studied extensively, and the consensus is clear: There are no cost or military readiness drawbacks associated with allowing trans people to fight for their country," Mr Block said. "The president is trying to score cheap political points on the backs of military personnel who have put their lives on the line for their country."

Washington Post, New York Times, Reuters