WASHINGTON, Dec. 29 -- U.S. President Barack Obama acknowledged the "real" threat posed by the Islamic State in an interview aired Monday, but ruled out another trillion dollars in aid in the efforts to fight the extremist group.

"I think we can't underestimate the danger of ISIL," the president told the National Public Radio (NPR), using another acronym for the group which has seized about one third of the territory in Syria and Iraq.

Citing the group's aspirations to control large swaths of territory and its possession of resources and an army, Obama said he saw "great dangers" posed to U.S. allies and the threat to destabilize entire regions.

"So I don't want to downplay that threat. It is a real one," he added.

Washington is leading air raids on targets of the Islamic State in both Syria and Iraq with a view to "degrading and ultimately destroying" the group, but acknowledges it would take time.

Obama told the NPR that he is "very hesitant" to devote another trillion dollars, as Washington has done in the 13-year Afghan war, to help in efforts to combat the Islamic State.

"Because we need to spend a trillion dollars rebuilding our schools, our roads, our basic science and research here in the United States, that is going to be a recipe for our long-term security and success," he explained.

"And what we've also learned is that if we do for others what they need to do for themselves -- if we come in and send the Marines in to fight ISIL, and the Iraqis have no skin in the game, then it's not going to last," he added.

U.S. and its North Atlantic Treaty Organization (NATO) allies are facing a violent insurgency by the Afghan Taliban as they formally ended their combat mission in the Asian country on Sunday.