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In the video showing journalist Steven Sotloff鈥檚 death at the hands of the Islamic State, the killer, wearing a black hood, claimed that the brutal beheading was retaliation for US airstrikes against IS forces.聽

Yet US officials have promised that the military airstrikes will not abate. Indeed, they have continued in the two weeks since the release of another video showing the death of another American journalist, James Foley.

President Obama condemned the killings during a press conference in Estonia Wednesday, saying that IS brutality toward American journalists will only strengthen US resolve to fight the group.

鈥淲hatever these murderers think they鈥檒l achieve by killing innocent Americans like Steven,鈥� he said, 鈥渢hey have already failed.鈥�

So, what has been the impact of airstrikes on IS, also known as ISIS and ISIL? Top Pentagon officials emphasize that there is only so much they can accomplish through airstrikes.

鈥淚f we鈥檝e learned nothing over 13 years of war, it鈥檚 you can鈥檛 completely eliminate extremism anywhere through simply kinetics, through airstrikes alone,鈥� Rear Adm. John Kirby said in a Pentagon briefing last week.

Instead, airstrikes can be used to 鈥渃ontain鈥� IS advances 鈥� 鈥渂ut not in perpetuity,鈥� warned Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, at a recent press briefing.

The US military has launched approximately 110 airstrikes to date against IS, at the cost of approximately $7.5 million a day.

They have been focused on one 鈥渧ery, very clear鈥� and limited mission, Rear Admiral Kirby said. 鈥淲e are there to support Iraqi and Kurdish forces as they take the fight to ISIL.鈥�

The majority of the strikes have been conducted in and around the Mosul Dam 鈥� a key source of water for the country.

Yet while the strikes drove IS fighters out of the area, 鈥淚SIL keeps wanting to take it back,鈥� Kirby said in the briefing last week. 鈥淭hey鈥檙e still under attack almost every day there at the facility.鈥�

This continues to be the case, since Iraqi security forces cannot hold the dam on their own, say senior Pentagon officials. 鈥淎s long as ISIL continues to threaten the facility, we鈥檒l continue to strike them,鈥� Kirby added Wednesday.

These airstrikes against IS formations in Iraq are 鈥渧ery useful in limiting territorial expansion and are essential to rolling back ISIS, because neither Iraqi security forces or the peshmerga鈥� 鈥� the Kurdish army in the north of Iraq 鈥� 鈥渁re capable of taking the offensive,鈥� says Christopher Harmer, senior naval analyst at the Institute for the Study of War in Washington. 鈥淏ut in terms of destroying ISIS, we鈥檙e not even in the ballgame.鈥�

Instead, the United States has 鈥渘ibbled at the periphery of ISIS. We鈥檝e nipped at their heels,鈥� adds Mr. Harmer, who served as deputy director of Future Operations for the US Navy鈥檚 Fifth Fleet in the region from 2008 to 2009.

From the perspective of IS leadership, halting the momentum of the group has not been a tragedy, he argues. 鈥淩ight now it鈥檚 not the worst thing in the world for them to stop taking new territory and consolidate the gains that they have. You can only process so many new fighters so quickly.鈥�

Meanwhile, IS is still on the offensive in Syria, 鈥済obbling up new territory,鈥� Harmer adds.聽

Mr. Obama is weighing the possibility of US airstrikes in Syria. In the meantime, he said last week that the airstrikes in Iraq were being used to 鈥渄egrade鈥� IS鈥檚 capabilities.

Military analysts jumped on this point, noting that 鈥渄egrade鈥� in military parlance is quite different from 鈥渄estroy.鈥�

Pentagon officials say they will not elaborate on the president鈥檚 comments. 鈥淚 don鈥檛 make policy here,鈥� Kirby told reporters last week, emphasizing once again that military strikes alone will not be sufficient to take on IS.

鈥淲hatever the options are for Syria, it鈥檚 not just going to be military,鈥� Kirby said. 鈥淚t can鈥檛 just be military. There鈥檚 not going to be a military solution here to the threat ISIL poses. It鈥檚 just not going to happen.鈥�