US military investigation has concluded that a series of 'unintentional human errors' led to a coalition air strike on September 17 that killed fighters aligned with the Syrian regime instead of the Islamic State militants they were targeting.

The incident, which Moscow said killed more than 60 Syrian soldiers, sparked a controversy and prompted an emergency United Nations Security Council meeting as tensions between Russia and the United States spiked.

Brigadier General Richard Coe, who led the investigation, told reporters at the Pentagon via a conference call on Tuesday that the major errors ranged from a basic mis-identification of targets to “group think” during intelligence development and even a communications blunder on a hotline with Russia.

But Coe also defended the coalition personnel involved, saying they were “good people trying to do the right thing.”

“These people get it right far more often than not, but this time they came up short,” said Coe.

The investigation threw light on the difficult ─ and dangerous ─ work of developing targets for coalition air strikes against Islamic State in parts of Iraq and Syria where the US does not have forces on the ground or reliable informants within the population to ensure its intelligence is sound.

"The US-led coalition mistook Syrian-aligned forces for Islamic State fighters in part because they were not wearing traditional uniforms," said Coe.

An early mistake ─ misidentifying a vehicle as belonging to Islamic State ─ coloured intelligence that came later when it drove into a larger fighting position near Deir al-Zor airport.

But Coe acknowledged that major red flags were missed. One analyst saw a tank moving around and even typed into a network chat room that “what we are looking at can't possibly be ISIL,” said Coe, using an acronym for Islamic State.

The mistakes continued even after the strike began.Moscow had reached out repeatedly through a hotline to the US-led coalition, trying to inform them that they were striking Syrian regime targets instead of Islamic State.

But the designated US military point-of-contact was unavailable for 27 minutes. In those 27 minutes, 15 of the strikes took place against what the U.S.-led coalition believed were Islamic State fighters.

“This was obviously a missed opportunity to be able to limit the damage of the mistake,” said Coe, adding that the strikes would have continued had the Russians not called and eventually passed along their information.

The strikes included aircraft from the United States, United Kingdom, Australia and Denmark which dropped 34 precision-guided weapons and fired 380 rounds of 30-millimetre ammunition.

Even before the strike took place, the coalition made a big mistake ─ it initially contacted the Russians to inform them that aircraft would be near Deir al-Zor, but gave the wrong coordinates for the strike.

“Of course we don't know for sure, but it is possible had we passed the right location to the Russians, they would have had the opportunity to warn us before the first strike even started,” Coe maintained.