In calling for global action against the "cancer" of Islamic State militants, President Barack Obama puts the fight against them at the top of his agenda in a move that also raises questions about his military strategy in Iraq.

Obama made it clear. The beheading of American journalist James Foley at the hands of the group that has captured swaths of Iraq and Syria shocks the world's conscience, and only reinforces Washington's determination to fight "this kind of nihilistic ideologies."

Obama had already insisted for days that the struggle to rout out IS will play out over the long term, and that there was no timeline for targeted US airstrikes.

And Washington has highlighted encouraging results obtained in close collaboration with Kurdish and Iraqi forces since in the more than 80 air raids conducted since August 8, such as the recapture of Iraq's largest dam.

"ISIL also has strengths and weaknesses," Pentagon spokesman Rear Admiral John Kirby said Tuesday, using one of the acronyms by which the group is known.

"They are potent. They are well-resourced. They are pretty well-organized for the terrorist network that they are."

But he also stressed that the militants were not invincible giants either.

"We've begun to see that through the use of these strikes, their morale is suffering; their competency and capacity has been damaged. And so they're not invincible either."

Defense policy expert Stephen Biddle of the Council on Foreign Relations noted, however, that challenges remained ahead, despite the success of the initial air strikes two and half years after American troops withdrew from the country.

"In this kind of war, the early airstrikes have an immediate effect partly in destroying targets that haven't taken adequate precautions," he said.

But once the fighters adapt, spread out their military equipment and mix in the civilian population, it's a major game-changer.

"That does not mean that the airstrikes are useless -- it makes ISIL equipment less effective compared to when there was no air threat," Biddle said.

"But once the enemy is taking counter measures and is not presenting you with this kind of ideal targets any more, you don't get the same payoff by doubling the amount of airstrikes," he added, calling for a medium- and long-term strategy to be clearly defined.

- No end in sight? -

While the Pentagon insists its goals are clearly defined, the definition itself -- delivering humanitarian aid and protecting US assets and personnel -- is open to interpretation.

And the scope has widened with the operations to recapture the Mosul dam and with sometimes dozens of airstrikes conducted in a single day.

The Obama administration justified the operations to retake the dam sprawled across the Tigris River by warning that if the militants compromised it, a huge wall of water could flood the northern city of Mosul and even the capital Baghdad, killing an untold number of civilians.

But nearly two weeks after the first US air raids in northern Iraq, many questions remain on how it is being carried out -- and about the scope of US military actions in the weeks and months to come.

"That, of course, remains the big unknown -- how far will President Obama go?" Max Boot of the Council on Foreign Relations wrote in Commentary magazine.

"Beyond protecting the Yazidis and retaking Mosul dam, we still need a strategy to annihilate ISIS. It can be done -- and if done right it will be the best, indeed the only worthy, response to James Foley's barbaric demise."

He called for sending 10,000 US soldiers and military advisors to Iraq, as well as for a significant reinforcement of the air raids.

Such a large deployment is not yet at hand in Washington, where officials repeatedly stress that ground troops are out of the question and that much of the solution lies in the hands of new Prime Minister Haidar al-Abadi, tasked with forming a unity government in deeply divided Iraq.

The State Department had asked for 300 more American troops to be sent to Iraq to protect US facilities, bringing to 1,150 the number of American soldiers and military advisers in the country.

On the domestic front, the release of Foley's execution video could make the American public sense a greater threat from IS, and the price it is willing to pay to confront it.

Republican lawmakers stress that while Obama's military response is justified, it has been far too shy so far.

"The more he delays and the more he acts incrementally, the more ISIS adjusts and the more difficult they will become," Senator John McCain told The Arizona Republic.

"And one of the decisions that he has to make is to attack ISIS in Syria because they are moving the captured equipment there and they are fighting there and their enclaves are there. They have erased the border between Iraq and Syria."