WASHINGTON — Partially lifting the secrecy that has cloaked one of the United States’ most contentious tactics for fighting terrorists, the Obama administration on Friday said that it believed that airstrikes it has conducted outside conventional war zones like Afghanistan have killed 64 to 116 civilian bystanders and about 2,500 members of terrorist groups.

The official civilian death count is far lower than estimates compiled by independent organizations that try to track what the government calls targeted killings, and human rights groups expressed doubts about the reliability of the government’s numbers. Most of the strikes have been carried out by drones in chaotic places like Libya, tribal Pakistan, Somalia and Yemen, though a small number have involved traditional aircraft or cruise missiles.

At the same time, President Obama issued an executive order making civilian protection a priority and requiring the government in the future to disclose the number of civilian deaths each year. The order, which could be canceled or altered by a future president, tries to commit his successors to greater openness than he has achieved in his first seven years in office.

For Mr. Obama, the drone program is personal, a defining feature of his presidency. As he expanded the use of drones far beyond what President George W. Bush had begun, he took a direct role in approving some strikes, including the deliberate killing of an American, and in setting up rules to govern them.