ISIS an 'Incredible' Fighting Force, US Special Ops Sources Say

— -- With the Obama White House left reeling from the "savage" slaughter of an American journalist held hostage by ISIS terrorists, military options are being considered against an adversary who officials say is growing in strength and is much more capable than the one faced when the group was called "al Qaeda-Iraq" during the U.S. war from 2003-2011.

ISIS, the Islamic State of Iraq and Syria, has been making a "tactical withdrawal" in recent days in the face of withering U.S. airstrikes from areas around Erbil in northern Iraq and from the major dam just north of Mosul it controlled for two nail-biting weeks, according to military officials monitoring their movements.

"These guys aren't just bugging out, they're tactically withdrawing. Very professional, well trained, motivated and equipped. They operate like a state with a military," said one official who tracks ISIS closely. "These aren't the same guys we fought in OIF (Operation Iraqi Freedom) who would just scatter when you dropped a bomb near them."

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ISIS appeared to have a sophisticated and well thought-out plan for establishing its "Islamic Caliphate" from northern Syria across the western and northern deserts of Iraq, many experts and officials have said, and support from hostage-taking, robbery and sympathetic donations to fund it. They use drones to gather overhead intel on targets and effectively commandeer captured military vehicles – including American Humvees -- and munitions.

"They tried to push out as far as they thought they could and were fully prepared to pull back a little bit when we beat them back with airstrikes around Erbil. And they were fine with that, and ready to hold all of the ground they have now," a second official told ABC News.

ISIS didn't necessarily count on holding Mosul Dam, officials said, but scored a major propaganda victory on social media when they hoisted the black flag of the group over the facility that provides electricity and water to a large swath of Iraq, or could drown millions if breached.

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U.S. special operations forces under the Joint Special Operations Command and U.S. Special Operations Command keep close tabs on the military evolution of ISIS and both its combat and terrorism -- called "asymmetric" -- capabilities, officials told ABC News. A primary reason is in anticipation of possibly fighting them, which a full squadron of special mission unit operators did in the Independence Day raid on an ISIS camp in Raqqah, Syria.

"They're incredible fighters. ISIS teams in many places use special operations TTPs," said the second official, who has considerable combat experience, using the military term for "tactics, techniques and procedures."

In sobering press conference Friday, Secretary of Defense Chuck Hagel said ISIS has shown that it is “as sophisticated and well-funded as any group that we have seen.”

“They’re beyond just a terrorist group. They marry ideology, a sophistication of strategic and tactical military prowess. They are tremendously well-funded,” he said. “This is beyond anything that we’ve seen.”