Washington - Defence Secretary Jim Mattis suggested on Monday that President Donald Trump's announcement that he was banning transgender people from serving in the US military may not be a done deal.In a series of three tweets last month, Trump upended an Obama-era policy of more than a year that allowed transgender troops to serve openly.But in the weeks since Trump's July 26 tweets, the White House has still not issued formal guidance to the Pentagon explaining how a ban on transgender personnel would work, or what would happen to those transgender troops who have already come out.Mattis said he had "no doubt" that the White House would be providing additional guidance on the issue, but said the Pentagon is giving military input."The policy is going to address whether or not transgenders can serve under what conditions, what medical support they require, how much time would they be perhaps non-deployable, leaving others to pick up their share of everything," Mattis told Pentagon reporters."There's a host of issues... it's obviously very complex."When pushed on whether the Pentagon will still employ transgender troops, Mattis said: "We are going to study the issue."In a lawsuit filed in federal court last week, five transgender women from the Air Force, Coast Guard and the Army said they faced uncertainty about their futures, including whether they would be fired or lose post-military and retirement benefits.The number of transgender troops among America's 1.3 million active duty service members is small, with estimates topping out at 15 000.Trump's tweeted announcement came with little apparent coordination with the Pentagon and landed while Mattis was on vacation.Several senior military officials have voiced unease over the policy shift, with the head of the Coast Guard saying he would not "break faith" with transgender personnel.Trump last week said he did the Pentagon a "great favour" by banning transgender troops, saying the issue had been "complicated" and "confusing" for the military.