A US military investigation has found that "unintentional human errors" led to a coalition air strike that killed Syrian forces instead of the intended target, so-called Islamic State (IS).

The attack which happened on 17 September killed over 60 Syrian fighters, according to the Russian army.

The strike was stopped when Russia informed the US that the target they were hitting was in actual fact Syrian forces and not the intended target of IS.

The airstrike included coalition aircraft from the UK, US, Australia and Denmark.

Investigating officer, Brigadier General Richard Coe said the major errors ranged from misidentification of targets to a communications blunder on a hotline with Russia.

The mistakes continued even after the strike began near Deir al-Zor airport.

Russia 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 U.S. military point-of-contact was unavailable for 27 minutes.

It was in those 27 minutes, 15 of the strikes took place against what the US-led coalition believed were so-called IS fighters.

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

But Coe also defended the coalition personnel involved, saying they were "good people trying to do the right thing." He added that "these people get it right far more often than not, but this time they came up short,"

The investigation threw light on the difficult and dangerous work of developing targets for coalition air strikes against IS in parts of Iraq and Syria.

So-far the war in Syria has taken the lives of over an estimated 400,000 civilians and fighters since the start of the conflict on 2011.

Around 20,000 people have fled the rebel-held eastern Aleppo in the past 48 hours as fighting has intensified, according to the Red Cross.

In a statement, the aid agency said that civilians must be allowed "safe passage" out of the eastern sector and that it stood ready to organise medical evacuations of sick and wounded.

In the past five months 60,000 people have left Syria's biggest pre-war city.