Mattis, speaking to reporters at the Pentagon, said that he and his staff are still studying the issue, including how having transgender service members affects other members of their units.

The Pentagon chief, asked whether transgender people now in the military will be forced out of their service, pointed to a statement that Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, issued a day after Trump’s announcement last month. Dunford said that openly transgender people will be allowed to continue to serve until there is guidance from the president on how to proceed.

“The chairman immediately went out and said immediately, ‘Everyone stand fast until we get the direction,’ ” Mattis said. “I understand that this is probably more about your suspicion about what could be coming, but the fact is, we have received no direction that would indicate any harm to anybody right now.”