NEW YORK -- The United States is expressing concern that Islamic State militants will use the populations of Iraq and Syria as human shields to protect themselves from US-led air strikes.The tactic was used by Saddam Hussein for shelter from Western forces during the Gulf War and, most recently, by Hamas during Israel's Operation Protective Edge in Gaza.Hours after strikes first began on the city of Raqqa, eastern Syria, where the militant group holds court among a population of 220,000 civilians, the Pentagon said that Islamic State militants had already begun "adapting" to its air campaign in Iraq by dispersing and hiding in civilian areas."It obviously is something that we prefer to do when collateral damage, or concerns about precision in a closed environment, in an urban environment... is in play," Lt. Gen. William Mayville Jr. said on Tuesday from the Pentagon. "There's obviously a desire to put something on the ground.""We have seen evidence that they're already doing that," Mayville continued, referring to the asymmetric technique, and calling Islamic State an "adaptive and learning" force.The use of human shields in Gaza by Hamas was acknowledged and condemned in July by the State Department, which called the tactic "deplorable." The imprecise nature of air strikes, and the prevalence of that stratagem, led in part to the ground invasion of Gaza by Israeli Defense Forces.In that effort, the Israeli government sought to destroy tunnels burrowed by Hamas into Israeli territory from Gaza, and to substantially diminish Hamas' stockpile of rockets and rocket launchers after a sustained barrage on Israel's southern communities.But after a third United Nations-run facility was shelled by Israeli forces during the 50-day conflict, the US reacted with forceful language: "The suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians," State Department spokeswoman Jen Psaki said.Jerusalem and Washington both classify Hamas and Islamic State as terrorist organizations.Asked by The Jerusalem Post last week whether the situations were comparable, State Department deputy spokeswoman Marie Harf said the two counter-terrorism operations were "wholly different.""I don’t want to compare the two in any way," Harf said. "We hold ourselves to a very high standard. We’ve asked the Israelis to do the same thing, and we’ve spoken up when we don’t think that they have."The US is prepared to stake out, or even actively disperse civilian populations in order to minimize civilian casualties, while also avoiding the use of a ground force, Harf said."If that means waiting until there are fewer civilians in a specific area to take some sort of counterterrorism operation, we’ve done that before," Harf continued. "The president’s been clear that when we undertake counterterrorism operations, we take civilian casualties very seriously, go to every length we can to prevent them, even if – and especially if – these terrorists are operating in densely-populated areas."But Mayville acknowledged the difficulty of counter-terrorism operations in urban areas. Raqqa is the most densely-populated location of interest to the US in its mission, and where Islamic State will be hardest to uproot.The Obama administration seeks to train moderate Syrian forces to fight for, win and hold ground with the help of allied air power, similar to a formula agreed upon in Iraq with the Iraqi armed forces, and Kurdish Peshmerga forces in its north."One of the reasons we take so much time, and we're so deliberate about it, is to make sure to the greatest extent possible that we avoid collateral damage," US Deputy National Security Advisor Tony Blinken said in an interview with CNN on Tuesday night.Unlike in Gaza, civilians in eastern Syria are not geographically bound from seeking refuge elsewhere. Over 2.5 million Syrians have fled across their country's borders, and millions more are internally displaced.During Protective Edge, IDF officials advised Prime Minister Benyamin Netanyahu and his cabinet that the destruction of Hamas would require the full-scale invasion of Gaza.US President Barack Obama has called for the destruction of Islamic State, estimated to be worth billions of dollars and with 30,000 men in its ranks. The self-declared capital of the group's "caliphate" is Raqqa, where air forces from the US, Saudi Arabia, Bahrain, Jordan and the United Arab Emirates opened their campaign against its assets on Monday night.Over 2,100 people were killed in Gaza this summer, of whom between 50-69 percent were civilians, depending on the source of the figures.Harf rejected the notion that civilian populations would serve as safe havens, that air power would be insufficient and that the destruction of the group was impossible without confronting the challenges Israel faced over five bloody weeks of summer."It's not an either-or proposition here," she said, adding: "We will stay with this fight no matter how long it takes."