Eric Fanning, the first openly gay Army Secretary, spoke out Thursday against President Donald Trump's proposed ban on transgender individuals serving in the military.

"You can imagine my shock and outrage when President Trump announced, via Twitter of all things, that transgender individuals would be banned from military service," Fanning said in a fundraising letter for the OutServe-SLDN [Servicemembers Legal Defense Network] advocacy group.

"I was appalled that a commander-in-chief would refer to thousands of people serving in uniform as 'burdensome' and 'disruptive,'" said Fanning, who served as the 22nd Secretary of the Army in the Obama administration and also previously served as undersecretary of the Air Force.

In July, Trump said in a Tweet that the U.S. will no longer "accept or allow transgender individuals to serve in any capacity in the U.S. military."

He followed that up with a formal notice to the Defense Department ordering a transgender ban in the military and directing Defense Secretary Jim Mattis to conduct a study on the effects and costs of having transgender personnel in the ranks.

Trump later told reporters at his Bedminster, New Jersey, estate, that "it's been a very complicated issue for the military, it's been a very confusing issue for the military, and I think I'm doing the military a great favor."

A Rand Corp. study has 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 and National Guard.

Advocacy groups and transgender personnel currently serving in the military immediately filed a series of federal suits against the ban. Lower court rulings have rejected the ban as discriminatory and ordered the military to begin the process of taking in transgender recruits on Jan. 1.

The Trump administration lost again last week in an appeal of the lower court rulings to the U.S. Circuit Court of Appeals for the District of Columbia.

In his letter, Fanning said, "To be clear, transgender individuals have always served in the armed forces. They are not a burden or a disruption. They are just as capable as anyone else and they should be afforded the opportunity to serve openly and with dignity."

-- Richard Sisk can be reached at Richard.Sisk@Military.com.