Caitlyn Jenner has challenged Donald Trump on his promise to fight for transgender Americans, after he said they'll be banned from the US military.

The 67-year-old reality TV star is a Republican party supporter and voted for Mr Trump in last year's election.

She's tweeted the president, telling him there are "15-thousand patriotic transgender Americans fighting for all of us".

This AM, @POTUS attacked trans people fighting for our country. Every American should be outraged. My full response: https://t.co/ALJwnvFwQg

— Caitlyn Jenner (@Caitlyn_Jenner) July 26, 2017

Meanwhile, The White House says details of how transgender people are going to be banned from the US military haven't been worked out yet.

The president hasn't confirmed what's going to happen to those already serving either.

His press secretary Sarah Huckabee Sanders has been explaining Mr Trump's reasoning: