As the Islamic State in Iraq and Syria (ISIS) continue to gain momentum across northern and western Iraq, U.S. officials have grown concern about the threat it poses to the country, even more so after the group's recent beheading of an American journalist.

Defense Secretary Chuck Hagel, during a briefing at the Pentagon on Thursday, said the ISIS is more dangerous than the al-Qaeda, referring to the Islamic State as an "imminent threat" that marries its ideology with sophisticated military and tactical prowess.

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He stressed, the U.S. is aware of the challenges it faces, adding that the "military's involvement is not over."

Based on intelligence assessments of ISIS' ability to acquire and hold territories, the group could become a long-term threat in the Middle East.

Moreover, intelligence agencies noted indications that the ISIS successes in the region have attracted a number of recruits from all over the world to join the group's campaign.

But some officials and experts believe the ISIS threat to the U.S. may be exaggerated and that its extreme brutality and radicalism may work against its pursuit of a caliphate.

Former National Counterterrorism Center deputy chief Andrew Liepman said that while the ISIS has an admittedly vast supply of resources at their disposal - money, weapons and people, among others - the U.S. tends to overreact when surprised, as it has been in this case.

Experts said a key distinction between the ISIS and a traditional terrorist group such as the al-Qaeda was the former's desire to expand its caliphate more than its desire to carry out acts of terrorism.

Although the al-Qaeda also aspired to create a caliphate, Bin Laden often said that goal would take a long time and could only be achieved under proper conditions.

On the other hand, ISIS has already declared the arrival of a caliphate and has named itself the "Islamic State."

Pentagon press secretary Rear Adm. John Kirby said the ISIS wants to strike at Western targets but that they do not have the capability to carry out an attack on the U.S. right now.

"Attacking the U.S. is not their first priority," Liepman said, referring to the conflict in the Middle East where ISIS is currently preoccupied against defending advances from the Kurdish troops and Syrian troops and rebels.

U.S. officials said the ISIS' brutal methods in governing seized territories, while currently effective, may create splinter groups that could weaken its power.

Experts speculate fear of the ISIS is partly driven by the lack of information about the group.

 Aside from details of the group's caliph - Abu Bakr al-Baghdadi - the U.S. knows little about its structure and leadership.

Now, the U.S. is firing up its counterterrorism systems to know more about the group.

"We must prepare for everything... and the only way you do that is to take a cold, steely, hard look at it and get ready," Hagel said.

TagsISIS, Chuck Hagel, counterterrorism, Al-qaeda, Abu Bakr al-Baghdadi, US