WASHINGTON - A Trump administration policy barring most transgender people from joining the US military went into effect on Friday.

For those wishing to join the US military, a diagnosis of gender dysphoria is now presumptively disqualifying under the new policy, a Pentagon official told reporters.

People currently receiving treatment for gender dysphoria may continue to serve, as will people who transitioned before the new policy came into force, according to the official.

Gender dysphoria "involves a conflict between a person's physical or assigned gender and the gender with which he/she/they identify," according to the American Psychiatric Association.

President Donald Trump announced his intention to ban transgender troops in 2017, citing high medical costs as a reason, before introducing measures that have met with pushback from within the military, criticism by rights activists and multiple lawsuits.

The US Supreme Court ruled earlier this year that the latest version of the administration's transgender military policy, which is not a full ban but a large step in that direction, could take effect pending the outcome of ongoing litigation.

The House Armed Services Committee, controlled by Democrats, said lawmakers from the party "will continue to fight against this bigotry and ignorance," slamming the policy as "a huge setback that shakes the faith our troops have in the country."

Trump's predecessor, Barack Obama, enabled transgender people to serve openly in the military in 2016.

The Pentagon has estimated that 9,000 out of a total of 1.3 million active duty personnel identify as transgender.