Washington (AFP) - Transgender troops can continue receiving transition-related medical treatment for now, the Pentagon said Monday, amid mounting challenges to President Donald Trump's effective ban on funding such procedures as well as enlisting new openly transgender personnel.

Trump blindsided the Pentagon in July when he tweeted that transgender troops, who had been allowed to serve openly under rules implemented by Barack Obama's administration, would be barred from the military.

The White House formalized the decree in an August 25 directive, but Defense Secretary Jim Mattis has ordered the current Obama-era policy remain in effect until officials conduct a review of how a ban might be implemented.

"First and foremost, we will continue to treat every service member with dignity and respect," Mattis said in the memo to service chiefs released Monday.

He said troops with a gender dysphoria diagnosis can receive Pentagon-funded sex-change procedures until March 22, when no more reassignment operations will be allowed.

Trump has said the ban would save on the "tremendous" medical costs and disruptions that transgender personnel could create.

But a Rand Corporation study said only a tiny portion of service members would ever seek gender transition affecting their deployability or health expenditure, adding between $2.4 million and $8.4 million in costs -- a fraction of the Pentagon's more than $600 billion budget.

Trump's ban has provoked outrage from rights groups and from both sides of the political aisle.

A bipartisan group of US senators on Friday introduced legislation that would protect transgender troops from being booted for their gender identity.

The measure was co-sponsored by Senator John McCain, a senior Republican figure who leads the Senate Armed Services Committee, so is likely to gain wide traction and raises the possibility that Trump's ban will be overturned before it ever can go into effect.

Mattis additionally underscored that no service member can be booted from the military based on their gender identity -- for now. Mattis's review is due to be submitted by February 21.

Trump's transgender ban has also sparked several legal challenges.

OutServe-SLDN -- which works to end military discrimination -- and civil rights litigators Lambda Legal have filed a lawsuit and retired admiral Mike Mullen, former chairman of the Joint Chiefs of Staff, has thrown his support behind the challenge.