The US-led air campaign against Daesh or self-styled Islamic State militants in Iraq and Syria is having a devastating effect on the insurgent forces. That is, if you believe the Pentagon's continuing tally of destroyed and damaged vehicles, facilities and troop formations.

But the official statistics are meaningless. Because US pilots are flying blind. To a great extent, they don't know what - if anything - they're hitting.

US drone strikes in Pakistan since 2001 have killed up to 3,976 people, according to the British-based Bureau of Investigative Journalism. As many as 965 of the dead were innocent civilians, according to the bureau's count.

Read: Obama apologises for hostage deaths in Pak-Afghan border strike

One US drone strike against an al Qaeda compound in Pakistan in January killed two innocent hostages: American contractor Warren Weinstein and Italian aid worker Giovanni Lo Porto.

The White House admitted to the mistake in April in a rare bout of transparency regarding drone attacks.

More so than in any recent conflict, American aviators soaring over Iraq and Syria rely on remote-controlled drones to spot targets for them. This is one consequence of President Barack Obama's policy barring US ground troops from any direct combat role.

The big problem? Drones make terrible spotters. Even after decades of development, America's unmanned aerial vehicles can provide only a narrow, grainy view of the battlefield.

“Contrary to popular belief,” Andrew Cockburn writes in his new history of military drones, Kill Chain: The Rise of the High-Tech Assassins, “the imagery tends to be fuzzy.”

A war waged mostly from the air still demands people on the ground, people with eyeballs whose visual acuity still beats a drone's sensors.

As long as Washington refuses to deploy human spotters against Islamic State, it won't know for sure whether air strikes are hitting the right people - or anyone at all.

On their face, the official Defense Department stats are impressive.

Between the first air raids in August 2014 and June 22, no fewer than 15,000 US-led strikes destroyed or damaged 7,655 militant targets in Iraq and Syria, including 98 tanks, 325 captured US-made Humvees, 2,045 buildings and 1,859 fighting positions, the latter presumably occupied by Islamic State troops.

US officials estimated that the air raids have killed around 1,000 militant fighters a month.

“The destruction of ISIL targets in Syria and Iraq,” the Pentagon claimed, using the administration's acronym for Islamic State, “further limits the terrorist group's ability to project terror and conduct operations.”

But there are good reasons to doubt the military's damage assessments and body counts - and its assertion that aerial attacks have badly hurt Islamic State.

With their narrow field of view from a fixed overhead perspective, the drones that provide the main view of the war zone convey an imprecise picture.

This is also true for the drones that help guide many of the bombs the manned war planes drop. The robot uses a powerful laser pointer to “designate” a target. A sensor on the bomb steers tiny wings that allow the munition to follow the laser all the way to the ground. But the bomb's accuracy is only as good as the drone's accuracy.

Drones can also fire their own missiles and drop their own bombs at targets they spot themselves, however imprecisely.

Over Iraq and Syria, the drones stay very busy as they introduce dangerous uncertainty throughout the complex campaign.

“We're involved in pretty much every engagement,” Colonel James Cluff, commander of the 432nd Wing, said of the US-led air campaign over Iraq and Syria. The 432nd, stationed at Creech Air Force Base just north of Las Vegas, controls most of the Air Force's roughly 300 Predator and Reaper drones.

For operations in Iraq and Syria, small crews apparently launch and land the drones from a base in Kuwait. Operators at Creech control the robotic aircraft via satellite, staring at banks of video monitors displaying the grainy images that the drones' sensors are seeing.

By relying solely on these drones to locate targets and guide bombs, US troops run the risk of hitting nothing. Or worse - mistaking civilians for fighters and sending bombs raining down on them.

US drone operations in Somalia and Yemen have also killed hundreds of noncombatants in the two countries.

As with the battle against Islamic State, these campaigns are strictly aerial. There are no US troops on the ground to verify targets.

The Pentagon does employ thousands of “joint terminal air controllers,” highly trained spotters whose sole job it is to locate targets for air strikes, first-hand. And then to help guide attacking warplanes via radio.

“You have to calculate so many different things with you and with the aircraft,” said then-Airman 1st Class William Chandler, a joint terminal air controller who served in Afghanistan in 2010.

“They're the glue that holds this all together,” explained then-Brigadier General Steve Kwast, Chandler's commander. “What the JTAC [joint terminal air controller] does is have a conversation between the aircraft and the ground commander to make sure our operations are sophisticated enough to solve problems without doing harm to the Afghan people.”

There are more than 3,000 US military advisers in Iraq, including joint terminal air controllers. But Obama has specifically barred them from going near the front lines, where they would do their best work using their own eyes to scan the battlefield.

During a visit last September to a military headquarters in Tampa, Florida, Obama stressed that the US forces in Iraq “do not and will not have a combat mission.”

Public opinion toward American troops' role in Iraq has whipsawed on this. In October last year, 52 per cent of respondents to a Reason-Ruppe poll said they opposed deploying US ground troops to fight Islamic State. By February, this had shifted.

A CBS poll found 57 per cent of Americans were in favor of deploying ground forces in a combat role.

Drones instead fill in for human spotters, with potentially deadly consequences for civilians. And disturbing implications for Washington's strategy in Iraq and Syria. How can the US government truly know whether it's winning the war against Islamic State if it doesn't know for sure who or what it's bombing?

The Pentagon is aware of the problem.

During a Senate hearing in May, Lieutenant General John Hesterman, commander of Air Forces squadrons in the Middle East, admitted that having joint terminal air controllers on the ground “would be helpful.” Hesterman's admission may be the military understatement of the year, as bombs rain down on people that remote drone operators, scrutinising grainy overhead video, decided must depict enemy fighters.