US President Barack Obama has asked the American public to prepare for a "long-term campaign" against the Islamic State as he outlined his administration's plan to "degrade and ultimately destroy" the jihadist group.

Obama acknowledged to reporters that ISIS and its ideology have already spread beyond the frontiers of Iraq and Syria, after he was updated on the military campaign against the group during a rare visit to the Pentagon, Fox News reported.

"To the American people, I want to say we will continue to be vigilant. And we will ultimately prevail," Newsmax quoted Obama as saying.

But he added: "This will not be quick. This is a long-term campaign. ISIL is opportunistic and is nimble." Obama always refers to ISIS (Islamic State in Iraq and Syria) as ISIL (Islamic State in Iraq and the Levant) for still unclear reason.

Obama said the public should brace for a "generational struggle" and that success depends on "Muslim communities, including scholars and clerics, rejecting warped interpretations of Islam and protecting their sons and daughters from recruitment."

The President also cited ISIS' losses in Iraq and Syria, saying the group "can be pushed back" in the region.

"In short, ISIL's recent losses in both Syria and Iraq prove that ISIL can and will be defeated," he said.

Obama said training local forces is crucial in sustaining the campaign against the group.

"The strong consensus is that in order for us to succeed long-term in this fight against ISIL, we have to develop local security forces that can sustain progress. It is not enough for us to simply send in American troops to temporarily set back organizations like the ISIL, but then as soon as we leave only to see that void filled again by extremists," he said.

"If we try to do everything ourselves all across the Middle East, all across North Africa, we'll be playing whack-a-mole and there'll be a whole lot of unintended consequences that ultimately make us less secure."

The President said the over 5,000 airstrikes against ISIS already conducted in Iraq, Syria, and North Africa as well as the effort of the "galvanised" Iraqi government after Ramadi's fall are signs of increased efforts against the Islamic extremist group which seeks to establish its own caliphate, CNN reported.

The US-led coalition is going after the "heart" of the terrorist group in terms of both monetary and human resources, he said.

He also asked Congress to confirm Adam Szubin as under-secretary of the Treasury Department for terrorism and financial intelligence, in order to boost current efforts to halt the flow of finances to ISIS.