They were killed in the volatile province of Jowzjan, the charity added, without revealing their nationalities or who was behind the incident.
"We can confirm that six of our colleagues were killed and two are unaccounted for in Jowjzan province," a spokesman for the International Committee of the Red Cross told AFP, adding that a statement would be released later Wednesday.
"We are shocked and devastated," he added.
He was released nearly a month later, but ICRC and local officials did not say how he was freed or who was behind the abduction.
"Devastated by this news out of #Afghanistan," ICRC president Peter Maurer said on Twitter about the latest incident.
"My deepest condolences to the families of those killed - and those still unaccounted for."
Aid workers in Afghanistan have increasingly become casualties of a surge in militant violence in recent years.
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