WASHINGTON — US Special Operations Command will take a new, leading role coordinating the Pentagon’s effort to counter weapons of mass destruction, reinvigorating a long-running debate about how the military should handle threats posed by everything from nuclear weapons to chemical agents such as sarin.

The decision was approved by President Obama at Defense Secretary Ashton B. Carter’s request in August but is still taking shape in the Pentagon and could be finalized in January, defense officials said. Numerous aspects of the mission to counter weapons of mass destruction will shift to Special Operations Command from US Strategic Command, which then-Defense Secretary Donald H. Rumsfeld assigned to the mission in 2005.

The decision means yet another job for Special Operations, whose elite troops have been used heavily by Obama to strike the Islamic State and other militant groups. The command will coordinate the development of a ‘‘coherent’’ Defense Department response to weapons of mass destruction, said a senior military official, who, like others, spoke on the condition of anonymity because of the sensitivity of the mission. Special Operations will not be granted any new legal authorities for the mission but will have new influence in guiding how the Defense Department responds to threats of weapons of mass destruction.

Pentagon spokesman Gordon Trowbridge said the decision shows how seriously the Pentagon takes countering weapons of mass destruction.

‘‘Changes to combatant command authorities are not undertaken lightly, and in this case, the change reflects careful consideration of how best to address what is clearly a national security priority,’’ Trowbridge said.

The move follows long-standing complaints that Strategic Command, which oversees space operations, missile defense, and nuclear missions, has not devoted enough personnel and emphasis to the job. It also comes as others question whether Special Operations has been given too much power, in part due to a decision first acknowledged by Carter in October to have Joint Special Operations Command, the most secretive part of the group, coordinate all US efforts to track foreign fighters globally.

One senior defense official who has worked on the mission to counter weapons of mass destruction said that Strategic Command ‘‘has rarely invested the necessary political and intellectual capital’’ to push for issues pertaining to countering weapons of mass destruction. The official attributed that to the Pentagon not having US forces designated specifically to countering weapons of mass destruction and an ‘‘overall low sense of priority as compared to its other missions.’’

The question, the senior defense official said, is whether Special Operations will effectively address all concerns about weapons of mass destruction given its ‘‘narrow interests’’ focusing on potential terrorism involving the weapons.

Trowbridge disputed those concerns, saying regional combatant commands, such as Central Command and Pacific Command, will continue to execute missions to counter weapons of mass destruction, with Special Operations synchronizing efforts across the Defense Department ‘‘just as [Strategic Command] has done.’’

A defense official with knowledge of the discussions said the decision amounts to a ‘‘rebalancing of priorities’’ as other missions under Strategic Command’s control, including cyberwarfare, take resources. Strategic Command commanders, the official said, are ‘‘busy cats, and they have huge responsibilities in terms of managing our strategic response.

‘‘They’re responsible for, if necessary, waging nuclear war,’’ the official said. ‘‘That’s a huge responsibility.’’

Special Operations has long had a hand in efforts to counter weapons of mass destruction and trains extensively to respond in case a nuclear weapon ends up in the hands of terrorists. According to the book ‘‘Relentless Strike,’’ a best-selling history of Joint Special Operations Command published last year, virtually every joint readiness exercise the unit ran by the 1990s involved the counterproliferation of nuclear weapons, with special attention paid to the seizure of weapons that enemies might hide underground.

That mission has continued to evolve.