Defense Secretary Jim Mattis approved a six-month delay in allowing transgender recruits to join the US armed forces to make sure the change in policy will not affect the 'readiness' or 'lethality' of the military.

Pentagon spokeswoman Dana White said in a statement on Friday that Mattis accepted a recommendation to defer transgender applicants until January 1.

In a memo obtained by the Associated Press, Mattis wrote: 'After consulting with the service chiefs and secretaries, I have determined that it is necessary to defer the start of accessions for six months.

'We will use this additional time to evaluate more carefully the impact of such accessions on readiness and lethality.'

Defense Secretary Jim Mattis, pictured, approved a six-month delay in allowing transgender recruits to join the US armed forces to make sure the change in policy will not affect the 'readiness' or 'lethality' of the military

Last week military service leaders rejected Army and Air Force requests for a two-year wait out of concern that a longer delay would cause criticism. Mattis, right, is pictured with National Security Adviser HR McMaster on June 26

Last week military service leaders rejected Army and Air Force requests for a two-year wait, AP reported.

According to the outlet, Mattis' memo is in agreement with their plan which showed their concern that a longer delay would cause criticism.

The Pentagon ended its ban on openly transgender people serving in the US military in 2016 under the Barack Obama administration, so many transgender people who had already been serving in the military were able to be openly transgender.

However, new recruits were not allowed to enlist.

The Pentagon was expected to start allowing new transgender recruits to begin enlisting this year, provided they had been 'stable' in their preferred gender for 18 months.

The prospect of a delay under President Donald Trump's administration alarmed transgender advocates.

'There are thousands of transgender service members openly and proudly serving our nation today... what matters is the ability to get the job done - not their gender identity,' said Stephen Peters of the Human Rights Campaign.

Last year, then-Defense Secretary Ash Carter cited a study by the RAND Corporation saying there were about 2,500 transgender active-duty service members and 1,500 reserve transgender service members.

Rand's figures were within a range, which at the upper end reached 7,000 active duty forces and 4,000 reserves.

The delay will not affect transgender troops who are already serving in the military openly.