The UN is investigating the crash in the Sahara desert of a cargo plane, which is thought to have been carrying cocaine from Venezuela.
Alexandre Schmidt from the UN drugs agency told the BBC that the plane could have carried up to 10 tonnes of cocaine, which has not been found.
West Africa has become a major transit point for Latin American cocaine.
But Mr Schmidt said the size of the plane which crashed in Mali, near Gao, came as "a complete surprise".
He said that the engine numbers were being checked to confirm the type of plane and whether it had originated in Venezuela.
The plane, made by Boeing, seems to have encountered problems after taking off from a remote air strip 15km from Gao and was ditched in the desert.
Mr Schmidt said the plane was then burnt, leaving little evidence for investigators.
No bodies were found at the crash site and it is not known what happened to the unloaded shipment
Most cocaine is transported from Latin America to West Africa by small planes or boats, before being trafficked on to Europe.