President Donald Trump said on Wednesday he would ban transgender people from the U.S. military, an action appealing to some in his conservative political base but sowing confusion about the fate of thousands of transgender service members.

Trump's surprise announcement, in a series of Twitter posts, drew condemnation from rights groups and some lawmakers in both parties as discrimination with purely political motives. But it was praised by conservative activists and some Republicans.

Trump spokeswoman Sarah Sanders said the administration has not yet decided whether transgender service members already in the military would be immediately thrown out, saying the White House and Pentagon would have to work that out.

The action, reversing Democratic former President Barack Obama's policy, halted years of efforts to eliminate barriers to military service based on sexual orientation or gender identity.

"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 tweeted, without naming any of the generals or experts.

"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," added Trump, who as a presidential candidate last year vowed to fight for lesbians, gays, bisexuals and transgender people.

Sanders said Trump had "extensive discussions with his national security team" and Defense Secretary Jim Mattis was informed after the president made the decision on Tuesday.

"This was about military readiness," Sanders told a briefing. "This was about unit cohesion. This was about resources within the military, and nothing more."

Some White House officials were caught by surprise. A senior administration official said Trump had been determined to act for a while, but the question was the timing, with advisers split on whether to conduct reviews before announcing the move.

The Pentagon earlier referred questions about Trump's decision to the White House.

The announcement at least temporarily changed the subject in Washington with Trump's administration mired in investigations into his presidential campaign's contacts with Russia and struggling to win approval of any major legislation.

It was not the first time Trump has targeted transgender people since taking office in January. In February, he rescinded protections for transgender students put in place by Obama that had let them use bathrooms corresponding with their gender identity.

Senate Armed Forces Committee Chairman John McCain - the most prominent veteran in Congress, who was a Navy pilot and prisoner of war during the Vietnam War - called Trump's announcement unclear and inappropriate until an ongoing Pentagon study on the issue is completed and reviewed by Mattis, the military leadership and lawmakers.

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

Obama's Pentagon last year announced it was ending its ban on transgender people serving openly, calling the prohibition outdated. The Defense Department had been expected to start allowing transgender people to begin enlisting this year. But Mattis on June 30 approved a six-month delay in allowing transgender recruits to join the military.


'POLITICAL POINTS'

American Civil Liberties Union attorney Joshua Block said Trump had rejected the "basic humanity" of transgender service members.

"There are no cost or military readiness drawbacks associated with allowing trans people to fight for their country," 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."

Obama's defence secretary, Ash Carter, last year cited a study by the RAND Corporation think tank saying there were about 2,500 transgender active-duty service members and 1,500 reserve transgender service members.

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

The House of Representatives' top Democrat, Nancy Pelosi, noted that a Pentagon-commissioned study determined the cost of providing medically necessary transition-related care involving transgender service members would amount to one-100th of 1 percent of the military's healthcare budget. The study put the cost at $2.4 million to $8.4 million a year of the more than $50 billion the Defense Department spends on healthcare.

"Once again, President Trump has shown his conduct is driven not by honour, decency, or national security, but by raw prejudice," Pelosi said.

Sarah Warbelow of the Human Rights Campaign advocacy group said Trump's action amounted to "discrimination on the basis of sex and identity," and was open to legal challenge under the U.S. Constitution's guarantee of equal protection under the law.

U.S. Representative Ileana Ros-Lehtinen, a Republican whose son is transgender, said on Twitter: "No American, no matter their sexual orientation or gender identity, should be prohibited from honour + privilege of serving our nation."

Chelsea Manning, the transgender Army soldier who served seven years in prison for leaking classified data, said Trump's action "sounds like cowardice." Transgender celebrity Caitlyn Jenner defended "patriotic transgender Americans" in the military and asked Trump on Twitter, "What happened to your promise to fight for them?"

But Vicky Hartzler, a Republican congresswoman, praised Trump for changing Obama's "costly and damaging policy."

The U.S. military's ban on gays serving openly in the armed forces ended under Obama in 2011 after Congress passed legislation in 2010 reversing a law dubbed "don't ask, don't tell" that had forced the ouster of thousands of service members and others to hide their sexual orientation.

The Pentagon under Obama also opened all combat roles in the military to women.

The U.S. military at times has been in the vanguard of social progress. Trump's action came on the 69th anniversary of Democratic President Harry Truman racially integrating the armed forces, years before the 1950s and 1960s civil rights battles.