US President Donald Trump has pulled back from his blanket ban on transgender people serving in the military, giving the secretaries of defence and homeland security greater discretion over the issue.

According to Defense Department memorandum, the new policy will allow some transgender members to serve while barring any who have been diagnosed with gender dysphoria or undergone a gender transition, and are willing to serve in their biological sex.

Mr Trump signed a memorandum in August that effectively barred transgender people from the armed forces, but it was blocked in court two months later.

Now, as with his efforts to ban travellers from Muslim-majority countries, which was repeatedly challenged in court, the president has returned to the issue with a modified version of the original move.

"I hereby revoke my memorandum of August 25, 2017, 'Military Service by Transgender Individuals,' and any other directive I may have made with respect to military service by transgender individuals," Mr Trump's new memorandum said.

Instead, Mr Trump sought to shift responsibility onto two of his cabinet secretaries.

"The Secretary of Defense, and the Secretary of Homeland Security, with respect to the US Coast Guard, may exercise their authority to implement any appropriate policies concerning military service by transgender individuals," the memorandum said.

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The document said that the two cabinet secretaries recommended that people diagnosed with gender dysphoria be banned from military service.

The Defense Department released a copy of a memo from Pentagon chief James Mattis dated 22 February that provided further details.

"Transgender persons who require or have undergone gender transition are disqualified from military service," it said.

Those who have not been diagnosed with gender dysphoria "may serve, like all other service members, in their biological sex."