The White House has not yet decided how it will implement the president’s ban on transgender people serving in the US military.
Mr Trump’s surprise Twitter announcement on Wednesday has been met with criticism from rights groups.
Spokeswoman Sarah Huckabee Sanders said the administration would work alongside the Pentagon to decide how to proceed.
It is not yet clear how the announcement will affect current transgender service personnel.
Transgender service members have expressed concern that they will be discharged, or unable to re-enlist.
The timing also appears to have caught defence officials by surprise.
Why was the ban introduced?
“The United States government will not accept or allow transgender individuals to serve in any capacity in the US military,” Donald Trump tweeted.
“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 that the decision came “after consultation with my Generals and military experts”.
Some US media outlets have questioned the spending justification. The Washington Post drew attention to an analysis that the US military spends almost $42m (£32m) a year on the erectile dysfunction medication Viagra – several times the total estimated cost of transgender medical support.
How would it be implemented?
Asked at a press briefing if troops on battlefields would be immediately sent back, Ms Huckabee Sanders said the policy had yet to be worked out.
“The decision is based on a military decision. It’s not meant to be anything more than that,” she said.
On Wednesday, the defence department referred all queries to the White House.
Defence Secretary James Mattis was only told of the decision after Mr Trump made his announcement.
However, Ms Huckabee Sanders said: “This was something that was the product of consultation.”