WASHINGTON — President Donald Trump is barring transgender people from serving in the military “in any capacity,” citing “tremendous medical costs and disruption.”

Trump’s announcement Wednesday morning on Twitter did not say what would happen to transgender people already in the military.

The president tweeted that after consulting with “Generals and military experts,” the government “will not accept or allow Transgender individuals to serve in any capacity in the U.S. Military.”

“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,” he added.

White House spokeswoman Sarah Huckabee Sanders offered little clarity about the policy at a press briefing. Asked what will happen to transgender troops currently serving, she said the Department of Defense and the White House will work together “as implementation takes place and is done so lawfully.”

She did not provide a timeline.

Sanders described the move as a “military decision.” She said Trump was concerned the current policy is “expensive and disruptive” and “erodes military readiness and military cohesion.” She said the secretary of defense was notified yesterday after Trump made the decision.

At the Pentagon, members of the staff of Defense Secretary Jim Mattis appeared to have been caught unaware by Trump’s tweets.

A Pentagon spokesman, Navy Capt. Jeff Davis, declined to answer questions about what Trump’s tweets mean for the current policy, including whether transgender people already serving in the military will be kicked out.

“Call the White House,” he said.

The White House press office did not immediately respond to a request for comment.

Sen. Tammy Duckworth, D-Ill., who lost both legs in combat, blasted the decision.

“When my Black Hawk helicopter was shot down in Iraq, I didn’t care if the American troops risking their lives to help save me were gay, straight, transgender or anything else,” Duckworth said in a statement. “All that mattered was they didn’t leave me behind. If you are willing to risk your life for our country and you can do the job, you should be able to serve — no matter your gender identity, sexual orientation or race. Anything else is discriminatory and counterproductive to our national security.”

Trump’s decision also drew immediate angry responses from groups that represent transgender service members.

Matt Thorn, executive director of OutServe-SLDN, which represents the LGBT population in the military, said thousands have been serving in the U.S. armed forces without causing any issues.

“It’s an absolute absurdity and another overstep,” Thorn said. He threatened legal action if Wednesday’s decision is not reversed.

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

Victoria Rodriguez-Roldan, director of the Trans/Gender Non-Conforming Justice Project, National LGBTQ Task Force, said Trump “has stood against the trans community with this decision and is harming lives for the sake of political gains.”

“The military is often the last resort for people who can’t find jobs because of discrimination,” Rodriguez-Roldan said. She said the transgender community “will not stop fighting” for justice.

Already, there are as many as 250 service members in the process of transitioning to their preferred genders or who have been approved to formally change gender within the Pentagon’s personnel system, according to several defense officials.

The Pentagon has refused to release any data on the number of transgender troops currently serving. A Rand Corp. study estimated that there are between 2,500 and 7,000 transgender service members on active duty and an additional 1,500 to 4,000 in the reserves.

Transgender service members have been able to serve openly in the military since last year, when former Defense Secretary Ash Carter ended the ban. Since Oct. 1, transgender troops have been able to receive medical care and start formally changing their gender identifications in the Pentagon’s personnel system.

Carter also gave the services until July 1 to develop policies to allow people already identifying as transgender to newly join the military. Mattis announced earlier this month that he was giving military chiefs another six months to conduct a review to determine if allowing transgender individuals to enlist in the armed services would affect the “readiness or lethality” of the force.

Key concerns include whether currently enlisted troops have had medical or other issues that cause delays or problems with their ability to deploy or meet physical or other standards for their jobs. Military leaders also wanted to review how transgender troops are treated, if they’re discriminated against or if they have had disciplinary problems, defense officials have said.

They were not authorized to discuss internal deliberations publicly so spoke on condition of anonymity.

Among those angered by Trump’s decision was Illinois Rep. Robin Kelly, who issued a statement saying the president’s policy change is “disgusting, stupid and runs counter to our American values.”

She added: “The President’s decision to tweet out a major military personnel decision shows his complete and utter disdain for those who wear the uniform. Decisions affecting the brave Americans who put their lives on the line for this country deserve more than 140 characters in his consistent tweet stream of hate and ignorance.”

Associated Press writers Darlene Superville and Vivian Salama and Sun-Times staff contributed to this report.