Airstrikes on September 17 by the US-led coalition, which killed 62 and injured 100 Syrian soldiers in Deir ez-Zor, were the result of an “unintentional, regrettable error,” the US Central Command has said.

The mistake was "primarily based on human factors," Brigadier General Richard 'Tex' Coe, who headed the CENTCOM investigation, told reporters in a teleconference on Tuesday.

Russia was notified of the planned strike – for the first time – using the “de-confliction” hotline, but was given the wrong location, Coe said. The target coordinates supplied by the US coalition were 9km (6 miles) off, he said.

When Russian officers called the hotline to report the strikes were targeting Syrian positions, they were kept on hold for 27 minutes because the US officer who was the designated point of contact was not available. The bombing continued in that interval, according to Coe, and stopped once the Russian message went through.

The airstrikes targeted Syrian forces near Deir ez-Zor the airport, a vital supply conduit for the enclave besieged by Islamic State (IS, formerly ISIS/ISIL) forces. Damascus has accused the US of deliberately targeting Syrian troops in order to scuttle the ceasefire negotiated in Geneva. The Pentagon has maintained the airstrike was an accident, and that IS was the intended target.

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“It was not an accident by one airplane; it was four airplanes which kept attacking the position of the Syrian troops for nearly one hour or maybe a little bit more,” Syrian President Bashar Assad told AP on September 22, adding that IS forces attacked the Syrian Army at the same time.

"The Syrian government puts all the responsibility for aggression on the US, as the facts show that it was a deliberate attack, but not a mistake – even if America says the opposite,” Syrian Foreign Minister Walid Muallem said in September at the UN General Assembly, adding that this act of “vile aggression” proved the US and its allies were “accomplices of Islamic State and other terror groups.”

The September 17 airstrikes unraveled the truce agreed to on September 9 in Switzerland by Russian Foreign Minister Sergey Lavrov and US Secretary of State John Kerry. The ceasefire envisioned the creation of US-Russian joint implementation centers, but the Pentagon refused to even consider the idea.

The Pentagon reiterated on Tuesday that it remains uninterested in working with Russia to identify IS targets.

"We have no plans at this point to cooperate with Russia in that way," US Department of Defense spokesman Peter Cook said in response to a question during a press briefing.

However, he noted the importance of the "de-confliction" hotline to prevent a repeat of the September 17 airstrikes.