The issue raises questions about what will happen to the detainee and others like him, given that President Barack Obama has ruled out sending any more terror suspects to Guantanamo Bay and the United States does not want to create a holding center for IS captives in Iraq.
The US defense official, who asked not to be named, confirmed a New York Times report that said a "significant" operative had been captured.
Officials told The Times that US interrogators were with the detainee at a temporary detention facility in Erbil in northern Iraq, and that he would eventually be handed over to Iraqi or Kurdish officials.
The Times said its sources declined to identify the detainee or say how much he had cooperated, but the official AFP spoke to said the captive was providing useful information that could yield leads to other IS operatives.
"They're getting good stuff from him," the official said.
The detainee was captured by elite special operations troops who deployed to Iraq in recent weeks and whom the Pentagon calls a specialized expeditionary targeting force, or ETF.
The Pentagon has until now been tight-lipped about the team's operations, saying that discussing missions puts the elite fighters at risk.
The Times said the 200-member special operations team is made up of many Delta Force commandos.
Signup to our newsletter and follow us on Facebook and Twitter!
Though it is small in number, it represents the first major US ground combat force since the official withdrawal of US troops at the end of 2011.
Another 3,870 or so US troops are in Iraq on a mission to train and support Iraqi forces fighting the IS group.
"The ETF has begun operations in Iraq. But we will not discuss the details of those missions when it risks compromising operational security," Pentagon spokesman Captain Jeff Davis told AFP.
"One of the goals of the ETF is to capture ISIL leaders," he added, using an alternative acronym for the IS group.
"Any detention would be short-term and coordinated with Iraqi authorities."
Defense Secretary Ashton Carter said Monday the ETF was an increasingly important component of the US-led coalition's 19-month-old campaign to defeat the IS group in Iraq and Syria.
"It's a tool that we introduced ... to conduct raids of various kinds, seizing places and people, freeing hostages and prisoners of ISIL, and making it such that ISIL has to fear that anywhere, anytime, it may be struck," Carter said.
Obama is trying to close the controversial Guantanamo Bay prison but is unlikely to succeed given staunch resistance in Congress.
"We are not back in the business of having long-term detainees," a second US defense official told AFP.