The US military will consider opening its doors to transgender people who are currently banned from service and can be booted out if discovered, Pentagon chief Ashton Carter said Monday.

The secretary of defense made the revelation as he unveiled a working group that will be studying the issue during the next six months.

"The Defense Department's current regulations regarding transgender service members are outdated and are causing uncertainty that distracts commanders from our core missions" he said in a statement.

As a result, the Pentagon "will create a working group to study over the next six months the policy and readiness implications of welcoming transgender persons to serve openly," he said.

In the United States, despite the 2011 repeal of the divisive "Don't Ask, Don't Tell" law, which banned gays from serving openly, transgender people have remained banned.

"At my direction, the working group will start with the presumption that transgender persons can serve openly without adverse impact on military effectiveness and readiness, unless and except where objective, practical impediments are identified," Carter said.

"We have transgender soldiers, sailors, airmen and Marines -- real patriotic Americans -- who I know are being hurt by an outdated, confusing, inconsistent approach," he added.

- 'Step in right direction' -

Democrat Adam Smith, ranking member of the House Armed Services Committee, applauded the statement as a good first step.

"Incorporating the presumption that transgender individuals can serve openly, without adverse impact on the military effectiveness and readiness, is a step in the right direction," he said.

"But it is long past time that we definitively and affirmatively make it clear that gender identity should have no bearing on an individual's ability to serve," he added.

For House Minority Leader Nancy Pelosi, a Democrat, this includes everything from "ensuring that service records are updated to reflect a service member's identity, to ensuring that no one is forced to leave the service because of that identity."

"For transgender service members, veterans and their loved ones, this represents a monumental step toward honoring their sacrifice, courage and strength," she said of Carter's announcement.

There are believed to be thousands of transgender people serving in the US armed forces, but under the current rules, if they are discovered the military is required to dismiss them.

"Transgender men and women in uniform have been there with us, even as they often had to serve in silence alongside their fellow comrades in arms," Carter said.

Chelsea Manning, the US soldier convicted of leaking a trove of secret documents to the anti-secrecy website WikiLeaks, is perhaps the best-known transgendered person to have served in the US military -- at the time living as a male, Bradley Manning.

Manning is serving out a 35-year sentence at the Fort Leavenworth military prison in Kansas for the massive leak, where she has told journalists that much of her life has been marked by a lonely anguish over her gender identity.

The Human Rights Campaign, an advocacy group that works for gay and transgender rights, heralded the breakthrough.

"We welcome and applaud the announcement by Secretary Carter that the military will at last conduct a comprehensive review of the outdated ban that has for far too long discriminated against qualified transgender Americans who simply want to serve their country," said HRC President Chad Griffin, in a statement.

"The time for ending the military's ban on transgender service is long overdue, and we are confident that the Pentagon's review of this discriminatory policy will find what many have come to know is true: Transgender Americans have every right to serve their country openly and honestly, and their sense of patriotism and duty is no less than any other service member's," Griffin said.

"Our military and our country will be stronger when this archaic policy is finally discarded and we look forward to that day."