Six Red Cross workers have been killed and two others are missing in northern Afghanistan, the international charity said Wednesday, underscoring the growing dangers faced by aid workers in the war-battered country.

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.

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-e-Sharif and the neighbouring volatile Taliban hotbed of Kunduz.

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 ," 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."

In April 2015 the bullet-riddled bodies of five Afghan workers for Save the Children were found after they were abducted in the strife-torn southern province of Uruzgan.