Robbers in Guinea have stolen blood samples believed to be infected with Ebola, the Red Cross in the West African nation has said.

The samples, stored in tightly wrapped vials tucked into a cooler bag, were in the care of a Red Cross courier who was among nine passengers sharing a taxi when three men on a motorbike robbed the vehicle and took the samples, said Saa Mamady Leno of the Red Cross in Gueckedou.

The robbers forced the passengers out of the vehicle on Wednesday, stole mobile phones, cash and jewelry, and fired into the air as they demanded the handover of the cooler bag, Leno said.

The courier was taking the blood samples on a 265km journey along winding rural roads from central Kankan prefecture to a test site in southern Gueckedou.

Faya Etienne Tolno, a spokesman for the Guinea Red Cross, said the aid group had a shortage of vehicles for transport, which explains why a taxi was used. No one was injured in the incident, which took place on a road known for banditry.

"We don't understand why they stole the blood sample. Perhaps they thought there was cash hidden in the flask," Tolno said.

Dr Barry Moumie, who heads patient care for the national Ebola response coordination committee, told the Associated Press news agency: "We have informed the security services. If these thieves handle this blood, it will be dangerous."

Authorities publicly appealed on national radio on Friday to the unidentified robbers to hand over the samples.

Ebola, which has killed more than 5,000 people in Liberia, Sierra Leone and Guinea, is spread primarily by contact with infected bodily fluids including blood, faeces and vomit.

The virus emerged in Guinea at the start of the year and has infected around 1,900 Guineans, killing almost 1,200.