President Donald Trump announced Wednesday that transgender people may not serve “in any capacity” in the US military, citing the “tremendous medical costs and disruption” their presence would cause.

In a decision he disclosed on Twitter, Trump said the military would not “allow or accept” transgender service members, reversing a policy begun by the Obama administration last year.

It wasn’t immediately clear what would happen to transgender service members now in the Army, Navy, Air Force and Marines. An estimated 2,500 to 7,000 transgender people are among the 1.3 million active duty service members.

A Pentagon spokesman described Trump’s Twitter post as “new guidance” rather than an order from the commander-in-chief, and said the Pentagon would issue “revised guidance” to the military “in the near future.”