Mr Mattis said he would create a panel of experts "to provide advice and recommendations" on the implementation of the measure.

Mr Trump justified the ban on grounds of cost and disruption.

But the move faces a legal challenge by transgender rights activists.

Mr Trump signed on Friday a memo reinstating a ban on transgender people serving in the military, which was lifted under President Barack Obama.

But he left Mr Mattis to decide whether those already in the armed forces would be allowed to continue to serve.

In a statement, Mr Mattis said that the panel of experts would include members of the departments of Defence and Homeland Security.

"Once the panel reports its recommendations and following my consultation with the secretary of Homeland Security, I will provide my advice to the president concerning implementation of his policy direction," he said.

"In the interim, current policy with respect to currently serving members will remain in place."

After the Obama administration announced last year that transgender people would be allowed to serve openly in the military, many active duty members came out.

Between 4,000 and 10,000 US active-duty and reserve service members are believed to be transgender.

Mr Trump's memo followed a series of tweets in which he made his surprise announcement last month.

In the document, the president also directed halting the use of government funds for sex-reassignment procedures unless it was necessary to "protect the health of an individual who has already begun a course of treatment to reassign his or her sex".

The Obama-era policy had included a provision for the military to provide medical help for service members wanting to change gender.