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.
Trump's decision 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.
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 if they meet physical, medical and other standards and have been stable in their identified genders for 18 months. Military chiefs recently announced a delay on allowing transgender people to enlist.
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.