Defense Secretary Jim Mattis, normally reluctant to speak publicly about American troops deployed around the globe, took a different tack this week on Capitol Hill.

Pressed by lawmakers, including Senator John McCain, Republican of Arizona and the chairman of the Armed Services Committee, about the Pentagon’s failure to provide more details about the Afghanistan war plan, Mr. Mattis divulged new information about more aggressive rules of engagement there.

But the changes he described have yet to be issued as orders to troops in the field, according to American service members and officials in Afghanistan. With the new rules caught in bureaucratic limbo, Mr. Mattis effectively telegraphed the military’s plans to the Taliban before they could be put into action.

Why Mr. Mattis chose to publicly discuss the rules of engagement — parameters that are classified to ensure the enemy cannot take advantage of their limitations — is unclear. The Pentagon did not deny that the changes had yet to go into effect, but a spokesman, Lt. Col. Mike Andrews, said he strongly disagreed “that the secretary said anything in public that would place forces on the ground at risk, or help the enemy.”