Additionally on Tuesday, 140 members of the House of Representatives wrote a letter to Trump urging him to reconsider his "ill-advised and indefensible" ban.
But in the meantime, Mattis said the current policy allowing openly transgender individuals already in the military to serve will stay in place.
He said he would create a panel of experts that would include members of the departments of Defence and Homeland Security "to provide advice and recommendations" on the implementation of the measure, TIME magazine reported.
As part of the implementation plan, the Secretary of Defense, in consultation with the Secretary of Homeland Security, shall determine how to address transgender individuals now serving in the United States military.
Joel Osteen: 'We never turned away' Hurricane Harvey flooding victims
The church opened its doors Tuesday and has been serving primarily as a distribution center for supplies to help flooding victims. But the main thing is that the city didn't ask us to become a shelter then. "That's what Lakewood is all about".
"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow..."
Replacing transgender service members would cost the USA 100 times more than the annual cost of providing them transition-related care.
Mattis also added that the focus of the decision "must always be on what is best for the military's combat effectiveness leading to victory on the battlefield". Trump tweeted, a day after he said "All options are on the table" for dealing with Pyongyang. Mattis may hope that Congress similarly steps in here to take this issue out of his hands-and to take on its political risks, too.
For the first time, transgender service members could serve openly, and several thousand people in the Army, Air Force, Navy, Marines and Coast Guard did so. ACLU attorney Chase Strangio pointed out that Mattis can only offer recommendations to the Trump administration and that he would be unable to shield transgender troops from being dismissed.
Some outlets have been calling this a "freeze" because now serving transgender soldiers will be allowed to stay in the military until March 23, 2018.
Trump's ban has been met with ire from transgender advocates and the American Civil Liberties Union, who have stated that the ban is discriminatory. The group found that transgender troops cost $8 million per year in medical care and that they had no negative effects on military readiness.
Trump gave the order on Friday, stating that he wants the ban implemented by 23 March, 2018.
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