The International Committee of the Red Cross has lost contact with four staff members and a veterinary assistant on board one of its vehicles in northern Mali.
The ICRC said the group was traveling between Kidal and Gao when it went missing Saturday.
"We are extremely worried about the fate of our colleagues," said Christoph Luedi, head of the ICRC delegation in Mali. "We're doing everything we can to locate them as quickly as possible."
"It's important not to speculate about what might have happened, although no possibility has been ruled out."