Sen. Bernie Sanders of Vermont is calling the U.S. drone strike in Afghanistan that mistakenly killed seven children "unacceptable."

The comments by Sanders, the progressive champion and runner up to now-President Biden in the race for the 2020 Democratic nomination, comes in the wake of an acknowledgment by the Pentagon that the attack was a "tragic mistake."

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At the time of the August 29 attack, the Pentagon said the strike had targeted an Islamic State suicide bomber amid the U.S. led evacuation of Americans and Afghan allies at Kabul’s international airport, during the final days of the U.S. withdrawal from the warn torn Central Asian nation.

The head of U.S. Central Command, Marine Corps General Frank McKenzie, the head of the U.S. Central Command, said that at the time he was confident the drone strike took out averted an imminent threat to U.S. forces at the airport.

But reports of civilian causalities quickly emerged and U.S. military leaders later concluded that the strike killed 10 civilians, including seven children.

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McKenzie told reporters on Friday that "our investigation now concludes that the strike was a tragic mistake."

The killing of the civilians in Kabul by a drone operated remotely from U.S. forces outside of Afghanistan is raising questions about President Biden’s ‘over the horizon’ capabilities of launching counter-terrorism strikes in Afghanistan from outside of the country.

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Asked on CBS News' "Face the Nation" on Sunday if is comfortable with the Biden administration’s ‘over the horizon’ policy, Sanders said "I certainly hope they understand what happened and make sure that never happens again. This is not only a human tragedy, it reflects on us before the entire world. It's unacceptable."