U.S. and coalition air forces are aiming for zero civilian casualties in airstrikes against the Islamic State in Iraq and Syria (ISIS), frustrating some lawmakers who say the military campaign is progressing too slowly.

While officials say they can never be absolutely certain of who’s on the ground, U.S. and allied forces are refraining from airstrikes against ISIS if there’s a risk of even one civilian casualty.

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“There’s a target of zero civilian casualties, so if there are civilian casualty concerns, we would continue to monitor a target or a potential target to see if there is a way to mitigate that,” said an Air Force official.

In practice, the strategy means that sudden developments on the ground can often force pilots to call off airstrikes. If a car suddenly drives up to an enemy checkpoint, for example, a strike would be delayed until it could be determined that no civilians were present.

The focus on protecting civilians is adding a wrinkle to the U.S. bombing campaign in Iraq and Syria, which is already operating without the help of “spotters” on the ground who can call in strikes on known ISIS targets.

“It’s insane. Seventy-five percent of those combat missions return to base without dropping a weapon,” Sen. John McCain (R-Ariz.), chairman of the Armed Services Committee, said on Tuesday. “The air campaign is totally ineffectual.”

Military officials acknowledge the 75 percent figure but say it’s comparable to past campaigns.

Rep. Martha McSally Martha Elizabeth McSallySenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-Ariz.), a retired Air Force colonel and former A-10 pilot and squadron commander, last week questioned the strategy for avoiding civilian casualties, calling it an “extreme constraint.”

“If we’re trying to avoid one civilian casualty in not hitting a legitimate target, we’re allowing the Islamic State to continue to commit atrocities and murder against the people on the ground,” she said.

Retired Air Force Lt. Gen. Dave Deptula, who ran air campaigns in Iraq and Afghanistan, said the zero-casualty standard is beyond what is required under the laws of war.

“What the law of armed conflict requires is that all ‘reasonable’ measures be taken to avoid collateral damage and civilian casualties,” he said.

“If ISIL are using human shields, and those very unfortunate individuals get killed, that responsibility and blood lies on the hands of ISIL,” Deptula said, using another acronym for the group.

Others defend the zero-casualty standard, saying it has become the new normal after wars in Iraq and Afghanistan, where winning over the local population was considered critical to success.

“That’s been true for about the last 10 years, by the way — you know, based on the way we do conflict,” Lt. Gen. John W. Hesterman III, the top commander of the coalition air war against ISIS, said earlier this month.

Still, Deptula acknowledged that the pace of the ISIS campaign, which is averaging 12 strike sorties a day, is a “drizzle” in comparison to previous air wars.

According to Deptula, the military averaged 1,241 strike sorties a day during Operation Desert Storm; 691 a day in the first month of the Iraq War; and 86 a day during the opening phase of the Afghanistan War.

“Last thing I heard was that we were only making 30 percent of sorties, so there’s clearly some issues with the airstrikes that need to be addressed,” Sen. Kelly Ayotte (R-N.H.) said on Tuesday.

A senior military officer told USA Today earlier this month there was a “directive” to avoid any civilian casualties. It is not clear who gave the directive, though Defense officials say any such order was not from Washington.

Defense Secretary Ashton Carter alluded to the restrictions during a House Armed Services Committee hearing last week, saying it was “not just a U.S. judgment.”

“I think the only limitation [that] the people managing the coalition air campaign have — and this is a coalition judgment, not just a U.S. judgment — is to try to avoid civilian casualties,” he said.

Military officials say it is important to maintain the support of the Iraqi government and coalition partners for the air war.

"How we conduct ourselves in warfare against a terrorist enemy intermingled within the society that they have infiltrated will have profound impacts on the way in which the U.S. is viewed in the Middle East and in this campaign," said Air Force Col. Ed Thomas, spokesman for the Chairman of the Joint Chiefs of Staff.

The U.S. has conducted 4,759 airstrikes against ISIS since Aug. 7, 2014, according to the operation’s Combined Joint Task Force, and has substantiated only two civilian casualties. Five other alleged casualty cases are under investigation.

For now, most coalition airstrikes are hitting targets where it is easy to determine that civilians are not present.

When the coalition identifies a target where it is not easy to rule out civilian casualties, it goes through an extensive review and vetting process. Those targets can take days or weeks to be approved, but only make up about 8 percent of airstrikes, according to the Air Force official.

Military officials say the main limitation on the airstrikes is not the risk of civilian casualties but the nature of the battlefield, and the difficulty of telling who is on the ground.

ISIS is adjusting to the air war, and has stopped traveling in large convoys. Last week, the commander of the coalition’s military task force said ISIS was using tunnels to avoid being targeted by air.

It can also be hard for coalition troops to tell the difference between enemy and friendly forces, reflected in the low airstrike numbers around Ramadi, where ISIS, Iraqi security forces, Shiite militia fighters and Sunni tribal fighters are battling for control.

“Nobody wants to kill civilians,” McSally told the Air Force Times after last week’s hearing.

“But we use military force in order to achieve a military objective and minimize civilian casualties. If we are deciding not to hit a legitimate target because there may be a civilian casualty, now we’ve turned that on its head.”

-- Updated at 1:45 p.m.