U.S. Defense Secretary Jim Mattis said Tuesday that transgender troops would continue to serve in the military while he studies an order by President Donald Trump banning them from entering service in the United States military.

President Trump announced in July via Twitter his intent to roll back a lift by his predecessor's, Barack Obama's administration on transgender people serving in any capacity.

"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow Transgender individuals to serve in any capacity in the U.S. Military. Out military must be focused on decisive and overwhelming victory and cannot be burden with the tremendous medical costs and disruption that transgender people in the military would entail. Thank you."

The President issued a formal memorandum last Friday on the issue, saying the ban should be in effect from March 23, 2018.

But the memorandum gave Mattis discretion on how to handle transgender people already serving in the military.

The administration was facing lawsuits by transgender groups and service members, and on Monday the American Civil Liberties Union also filed a suit on behalf of several transgender troops challenging Trump's order.

In a statement Tuesday Mattis appeared cautious about implementing Trump's order.

He said the Department of Defense would establish a panel of experts and develop an implementation plan with a focus "on what is best for the military's combat effectiveness leading to victory on the battlefield."

In what appeared to be a barb against the slow movement in the White House to approve all the senior staff he needs, Mattis said "the soon arriving senior civilian leadership of the DOD will play an important role in this effort."

In the meantime, he said, existing policy on currently serving transgender troops would not change.

On Monday Pentagon officials declined to say whether there had been any studies or anecdotal reports on the impact of transgender people in the military, and they also declined to reveal any estimates of the number of transgender troops. Estimates run from the low thousands to as many as 15,000.