President Donald Trump has signed a memorandum that directs the US military to not accept transgender individuals as recruits and halts the use of government funds for sex-reassignment surgeries for active personnel unless the process is already under way.

The memo, released by the White House on Friday, laid out in more detail a ban on transgender individuals serving in the US armed forces that Trump announced via Twitter last month.

The announcement reversed a policy shift started under his predecessor, President Barack Obama.

In the memorandum, Trump directed the Department of Defense and Department of Homeland Security to stop using government funds for sex-reassignment procedures unless it is necessary "to protect the health of an individual who has already begun a course of treatment to reassign his or her sex".

The order requires Secretary of Defense Jim Mattis to determine in the coming months how to handle transgender individuals currently serving in the military using criteria including "military effectiveness and lethality," budget constraints and law.

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A White House official who briefed reporters about the memo declined to specify whether transgender individuals who are currently active in the military could continue to serve based on such criteria.

The official said Trump decided the Obama administration had not identified a sufficient basis for changing what was then long-standing policy on transgender troops.

The memo called on Mattis to submit his plan on how to implement the changes by February 21 of next year.

'Nothing less than a purge'

Trump's decision appealed to some in his conservative political base while creating uncertainty for thousands of transgender service members, many of whom came out after the Pentagon said in 2016 it would allow transgender people to serve openly.

According to the National Centre for Transgender Equality, there are about 15,000 transgender people currently serving in the US military.

The change drew swift criticism from advocates of lesbian, gay, bisexual and transgender (LGBT) rights.

"The President's order to remove transgender service members from the United States armed forces and deny them healthcare is nothing less than a purge," Matt Thorn, executive director of OutServe-SLDN, a group dedicated to LGBT equality in the military, said in a statement.

Jennifer Levi, an official at the gay rights group GLAD (GLBTQ Legal Advocates & Defenders), said that the US military "is strongest when all people who are fit to serve have the opportunity to do so".

"This unprecedented policy amounts to a purge of qualified, contributing troops, and will serve only to undermine unit cohesion and weaken military readiness."

Others online, including Chelsea Manning, a transgender army private who was jailed for one of the largest leaks of classified documents in US history, also criticised Trump's move, calling it a "shameful" and "hateful".

dumped news, friday evening ? 🤔 this isnt about trans people or military, its about normalizing hate 😎🌈💕 #WeGotThis https://t.co/THBJBdyROA — Chelsea E. Manning (@xychelsea) August 26, 2017

We've said this before and we will fight forever for it:@realDonaldTrump: TRANS PEOPLE ARE NOT A DISRUPTION.https://t.co/DtdjgJF8W2 — Women's March (@womensmarch) August 25, 2017

Donald Trump just officially signed the order banning transgender folk from the military.



Shame on him and shame on America. — Shaun King (@ShaunKing) August 25, 2017

Salute to all the transgender people in the military and those who have served. We are forever grateful for your service to our nation. — Michael Skolnik (@MichaelSkolnik) August 25, 2017