The change in the rules was first reported on Tuesday night by The Wall Street Journal, which portrayed it as providing new legal authority to the military.

A senior administration official, who spoke on the condition of anonymity because the directive is classified, said the government considers the underlying legal authority to be unchanged. The official said the administration had overlaid policy standards for approving strikes on top of that legal authority because the United States wants to carry out offensive attacks only against significant terrorists, not bomb groups “willy nilly.”

The change does not affect targeting procedures in other places like Libya where the Islamic State has a growing presence, the official said.

The Islamic State evolved out of the Qaeda affiliate that had battled American troops during the Iraq war and was eventually pushed into rebel-held regions of Syria. In 2013, after a dispute over tactics with Osama bin Laden’s successor as the leader of the original Al Qaeda, based in Pakistan, the Islamic State split from Al Qaeda and rebranded itself.

Declaring itself a caliphate, or global Islamist nation, it swept back across Sunni territory of Iraq. In August 2013, when Mr. Obama ordered the military to begin bombing its forces, the administration declared that it had legal authority to do so under Congress’s 2001 authorization to use force against the perpetrators of the Sept. 11, 2001, terrorist attacks and those who aided them — meaning Al Qaeda and the Taliban.

Stretching the existing legal authorization to cover the Islamic State was initially controversial since it had not only split from Al Qaeda but was also battling it. Congress has essentially acquiesced to that interpretation, however, by continuing to fund military operations against the Islamic State without enacting any separate authorization to use force against it.

The 2001 use-of-force authorization is also the basis of the United States’ continuing military operations in Afghanistan. Although the Obama administration says the American “combat mission” ended, it has kept troops there to train Afghan forces that are trying to fend off a Taliban resurgence, and to carry out strikes aimed at transnational terrorist groups like Al Qaeda. Some American combat deaths have continued.

Separately, the State Department last week designated the Islamic State’s affiliate in Afghanistan a foreign terrorist group, subjecting it to sanctions. The senior administration official said the timing was a coincidence.