Iraqi security forces inspect the site of three explosions in the New Baghdad neighborhood Wednesday. Reuters

Determined to counter fears that America may find itself bogged down in another war, the Obama administration insists the U.S. won't send troops to fight the Islamic State in a “combat role” in either Iraq or Syria. Echoing President Barack Obama's comments, Secretary of State John Kerry said Wednesday there has been no discussion to send ground troops to Syria “at this moment.” But experts wonder if the threat posed by the Islamic State can be eradicated by a second air campaign in Syria.

The U.S. is currently fighting the militant group known as the Islamic State of Iraq and Syria (ISIS) in both Iraq and Syria using airstrikes and has sent over 1,200 U.S. troops to Iraq. The U.S military personnel there are not combat troops but "military advisers" sent to help Iraqi and Kurdish forces, says the White House. On Wednesday, Congress approved additional funding to arm and train moderate Syrian rebels who are supposed to be America's allies on the ground.

The refusal to use combat troops -- to put boots on the ground, as the phrase goes -- means that the U.S. must win from the air, coordinating with those allies in country.

Airstrikes have their benefits, the biggest being a much lower casualty rate for U.S. troops. So far in Iraq, the U.S. has been using high-flying fighter jets and drones to drop bombs on the militants, which greatly minimizes the risk of planes being shot down and pilots getting captured.

But airstrikes have their limitations. They run the risk of significantly increasing civilian casualties. While ISIS has some known strongholds -- in Raqqa, Syria, for example -- which could be neutralized by airstrikes, their other hideouts will not be as easily targeted. Last week, the CIA estimated that the militants could have up to 31,500 fighters sprinkled across Iraq and Syria. They operate in clandestine cells in many parts of Iraq, Syria and elsewhere. The more the U.S. bombs them from the sky, the deeper they go underground: That could make the air campaign less effective while also endangering more civilians.

“If the U.S. only uses air power, ISIS will eventually hide in the cities and the U.S. will be faced with causing a lot of civilian casualties to get the group out or kill its fighters,” said Dr. Ivan Eland, author of the "The Failure of Counterinsurgency: Why Hearts and Minds Are Seldom Won."

Ground troops, on the other hand, would involve a significantly higher casualty rate for the U.S. As of April 2014, more than 6,800 American men and women have died in Iraq and Afghanistan since the U.S. led a ground invasion in 2003. Ground forces in either Syria or Iraq would also be very expensive. The 2003 Iraq and Afghanistan invasion will ultimately cost taxpayers $4 trillion to $6 trillion, including the cost of treating veterans, according to a Harvard study published last year.

The Pentagon has said that a ground attack could be necessary to successfully eradicate the militants while minimizing civilian casualties. Gen. Martin Dempsey told a Senate panel Tuesday that he's not ruling out the possibility of troops on the ground. But Obama's and Kerry's comments the following day were clearly intended to walk back any perception that combat troops would be sent.

“The U.S. military is right that you can't win without troops on the ground," says Eland. "The problem is that despite technological military superiority of great powers, including the United States, it doesn't count for very much. Counterinsurgency is mostly political. So it is actually better to use local forces on the ground, despite their technological inferiority. The use of U.S. ground forces to fight ISIS is unlikely to be successful.”

So far, Obama’s answer to having someone on the ground verifying targets is to arm and train rebel forces in Syria (now with congressional funding) as well as Iraqi and Kurdish allies in Iraq.

The problem is that in Iraq and Syria, the U.S. doesn’t have much to work with on the ground. The Kurdish peshmerga fighters have been successful in pushing back ISIS militants. But in Syria, the groups of moderate rebels that Congress has agreed to fund are much less trustworthy and much less effective, experts said, because their immediate enemy is not Islamic State. Last week, certain moderate and Islamist rebel groups in a Damascus suburb in Syria signed a “non-aggression” agreement with ISIS to focus on fighting President Bashar Assad’s forces.

Whatever its tactical limitations, though, the "no combat troops" plan is likely to remain firm, at least for the foreseeable future. Polls show that the American public is not willing to enter another ground war. According to an NBC News/Wall Street Journal poll, 61 percent of voters believe military action against ISIS is in the best interest of the U.S., but 40 percent think it should be limited to airstrikes.

"It's for me a blanket 'no,'" said Rep. Nancy Pelosi (D-Calif.) on Tuesday. "I don't think the American people are up for it."