Two other Red Cross workers were missing following the ambush in the volatile province of Jowzjan, in one of the worst attacks on the international charity in years.
Their convoy, comprising three drivers and five field officers, came under fire while they were carrying much-needed livestock materials to a remote area badly affected in recent days by heavy snowfall, Red Cross said.
"This is a despicable act," said Monica Zanarelli, head of the International Committee of the Red Cross in Afghanistan.
No militant group has so far claimed responsibility for the killings, but Jowzjan police chief Rahmatullah Turkistani blamed local Islamic State fighters.
"Daesh fighters are active in the area," he said, using the Arabic acronym by which IS is commonly known in Afghanistan.
"We had previously repeatedly warned them not to go to such dangerous areas under Daesh control."
Turkistani said the bodies of the six workers had been brought to a provincial hospital.
Some of the bodies had multiple bullet wounds and had been shot from close range in the head and chest, Fraidoon Habib, director of the hospital, told AFP.
"These staff members were simply doing their duty, selflessly trying to help and support the local community," Maurer said.
"Our thoughts are with the families and loved ones of our colleagues killed and those unaccounted for."
The Taliban, the largest militant group in Afghanistan, said they were not behind the attack.
Jowzjan is infested with insurgent groups and rife with armed struggles between competing pro-government militias.
The killings come after a Spanish employee of the ICRC was abducted on December 19 when workers from the charity were travelling between the northern city of Mazar-i-Sharif and the neighbouring volatile Taliban hotbed of Kunduz.
The ICRC, which has been operating in Afghanistan for decades, did not say how the latest incident would impact them.
"At this point, it's premature for us to determine the impact of this appalling incident on our operations in Afghanistan," Zanarelli said.
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