They have been detained in connection with an alleged coup attempt in Equatorial Guinea.
The men and three air crew were seized and their cargo plane impounded when it landed in Harare on Sunday.
The company which chartered the aircraft said the group was heading to the Democratic Republic of Congo to provide security services for an internationally owned mine.
However, both South Africa and Zimbabwe support the government of Equatorial Guinea, which says the group was part of a plot to overthrow the country's President.
