Updated 12:04 p.m. EST

President Donald Trump on Wednesday announced a ban on transgender people from serving in the U.S. military.

In a series of tweets, the president said, "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."

He added, "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. Thank you."

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The Pentagon didn't immediately release additional details on the president's announcement and referred all questions about the matter to the White House.

Earlier this month, a spokeswoman for Defense Secretary Jim Mattis said he was giving the service chiefs another six months to review whether allowing transgender individuals to serve would hurt the "readiness or lethality" of the force, The Associated Press reported.

The Army and Air Force had requested a two-year wait, while the Navy had requested a one-year wait in part to accommodate a request from the Marine Corps, according to the AP.

The delay ordered by Mattis did not affect transgender troops who were already serving openly in the military, according to Pentagon spokeswoman Dana White. It wasn't immediately clear what will happen to these individuals.

Some 250 people are transitioning genders or have been approved to change gender within the Pentagon's personnel system, the AP and other news organizations have reported.

The push to allow transgender troops to serve openly began during the administration of former President Barack Obama. Then-Defense Secretary Ashton Carter in the summer of 2016 directed the chiefs to draw up plans to accept transgender recruits and have them ready by July 1, 2017.

In 2011, Obama ended the military's "don't ask, don't tell" (DADT) policy that prevented gay and lesbian troops from serving openly.

The total number of transgender people in uniform is estimated to be less than half of 1 percent of the force, according to Rand Corp., a think tank based in Santa Monica and funded in part by the U.S. government.

There are between 1,320 and 6,630 transgender troops currently serving on active duty, which amounts to between 0.1 percent and 0.5 percent of the 1.3 million-member active component, and between 830 and 4,160 in the Selected Reserve, according to a 2016 study by Agnes Gereben Schaefer, a senior political scientist at the nonprofit. Advocacy groups, meanwhile, put the estimate at closer to 15,000 transgender troops in the ranks.

In any given year, a smaller number of those active-duty personnel, or between 29 and 129 service members, would likely seek gender reassignment surgery that could impact their ability to deploy, according to Schaefer.

While the number of transgender troops is relatively small, they serve in a variety of positions within the military.

In a story first reported last week by Military.com, one of the first female candidates for Navy special operations jobs may be transgender. A petty officer in the Navy's all-enlisted Special Warfare Combatant-Craft Crewman program, or SWCC, notified their chain-of-command that they identified as being transgender.

It wasn't immediately whether troops who already identified as transgender to their chain of command would face personnel action.

According to Navy policy guidance released last fall, a sailor must receive a doctor's diagnosis of medical necessity and command approval to begin the gender transition process, which can take a variety of forms, from counseling and hormone therapy to surgery. Sailors must also prove they can pass the physical standards and requirements of the gender to which they are transitioning.

The Rand report describes the cost associated with providing gender reassignment surgery and other medical care to transgender troops as "relatively low."

Providing health care coverage to transgender personnel would marginally increase the active component's health care costs by between $2.4 million and $8.4 million annually -- raising overall health care costs by between 0.04 percent to 0.13 percent, according to the study.

Lawmakers, advocates and others quickly reacted to the news of the president's announcement.

Sen. Kirsten Gillibrand, a Democrat from New York and a member of the Senate Armed Services Committee, described the move as "outrageous."

"This morning, transgender service members put on uniform and showed up to their military duties only to be told (on Twitter, no less) that their Commander in Chief doesn't want them in any capacity -- it's outrageous!" she said. "This new decision is harmful and misguided. It weakens -- not strengthens -- our military. And I’ll do everything in my power to fight it."

Sen. John McCain, a Republican from Arizona and the panel's chairman, said Trump's tweet "is yet another example of why major policy announcements should not be made via Twitter."

The 80-year-old senator, who was recently diagnosed with brain cancer, added, “The Department of Defense is currently conducting a study on the medical obligations it would incur, the impact on military readiness, and related questions associated with the accession of transgender individuals who are not currently serving in uniform and wish to join the military. 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."

Tony Perkins, a Marine veteran and president of the Family Research Council, a Christian public policy ministry in Washington D.C., praised Trump's move.

"I applaud President Trump for keeping his promise to return to military priorities -- and not continue the social experimentation of the Obama era that has crippled our nation's military," Perkins said in a statement. "The military can now focus its efforts on preparing to fight and win wars rather than being used to advance the Obama social agenda."

Aaron Belkin, director of the Palm Center in San Francisco, which promotes the study of lesbian, gay, bisexual, and transgender people in the armed forces, described the president's announcement as "a shocking and ignorant attack."

"As we know from the sad history of that discredited policy, discrimination harms military readiness," Belkin said in a statement, referring to DADT. "This is a shocking and ignorant attack on our military and on transgender troops who have been serving honorably and effectively for the past year."

-- Richard Sisk and Hope Hodge Seck contributed to this report.

-- Brendan McGarry can be reached at brendan.mcgarry@military.com. Follow him on Twitter at @Brendan_McGarry.