Speaking at an event in Washington, he said that Obama approved the change to let the U.S. commander "anticipate situations in which the Afghan security forces would benefit from our support."
"This is using the forces we have here in a better way," he told the Defense One Tech Summit, calling the expanded authority "a good use of the combat power that we have there."
The new authority for U.S. troops would pave the way for military operations to once again support conventional Afghanistan forces against the Taliban under limited circumstances, a senior U.S. military official told CNN ahead of Carter's public comments.
"This is not a blanket order to target the Taliban," the official said.
However, under the new authorities, U.S. troops would be allowed to accompany conventional Afghan forces into the field to advise and assist them. Under current rules, U.S. participation is largely limited to supporting Afghan special forces.
A U.S. official said the new plan allows mainly for "close air support" strikes to ensure the safety of U.S. and Afghan forces by hitting Taliban positions. But those airstrikes may also require U.S. military "air controller" personnel on the ground to pick out targets and relay details to pilots overhead.
The new rules could also open the door for more U.S. airstrikes and ground action against the Taliban to ensure U.S. and Afghan forces are protected, the official indicated. Still, the Pentagon does not believe U.S. troops will be "in direct combat," the official said.
The Pentagon and White House have been debating for weeks not only about a change to U.S. troops levels in Afghanistan, but whether to amend the current military authorities that restrict U.S. airstrikes against Taliban targets.
As of now, the U.S. military broadly has authority to strike any targets on the ground under three scenarios: to protect U.S. forces on ground; to go after the remnants of Al Qaeda; and to protect Afghan forces when they are facing imminent danger of being overrun by the Taliban.
The discussion inside the administration has been centered around whether to change those authorities so U.S. warplanes could now strike Taliban targets even if they do not pose a direct imminent threat.
There are about 9,800 U.S. troops in Afghanistan. The plan now calls for a drawdown to about 5,500 in 2017. The drawdown could be slowed within that time frame to keep more troops in the country for as long as possible, but it's not clear that decision has been made.
Broadly speaking, the U.S. believes the performance of Afghan security forces has improved, according to Brig. Gen. Charles Cleveland, the chief spokesman for the U.S.-led coalition. But there are concerns as the summer fighting season goes on that the Taliban will pose security problems in southern Afghanistan.
"The Taliban has shifted their main effort down to Helmand," Cleveland said, noting, however, that the U.S. hasn't seen the full Taliban offensive it expected. Still, the reemergence of the Taliban threat has led to this vigorous discussion inside the administration about what to do about it, officials told CNN.
The Defense Department had been expected to announce on Friday changes in the U.S. military operation in Afghanistan to increase the ability to attack Taliban targets, but it was canceled on Thursday according to several officials familiar with the effort. None of the officials CNN spoke to report to Carter, and no one agreed to be identified due to the sensitivity of the discussions inside the administration.