National Liberation Army (ELN) guerrillas earlier identified their captives, who have been held for over three months, as "Messrs. Breur Uwe and Breuer Gunther Otto, of alleged German nationality."
The men were captured in the Catatumbo region, in northeastern Colombia near the border with Venezuela, the group said, claiming they were spies.
Monday, however, the ELN asked German authorities to intercede with the hostages' kin.
"In relation to Messrs. Breur Uwe and Breuer Gunther Otto, ... we request that their family members send us real evidence to show their relationships to the gentlemen being detained," the new statement posted on the same site said.
And "in order to settle this situation, we ask the German government to name one of its accredited officials in Colombia to handle the paperwork between its government and the ELN," it added.
The rebels also requested information about the work histories of the men, apparently German brothers aged 69 and 73.
The ELN, is Colombia's second largest guerrilla group after the better-known Revolutionary Armed Forces of Colombia (FARC).
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Founded four decades ago, the ELN has around 2,500 fighters and is active in northeastern Colombia.