Jorge Enrique Botero says the International Red Cross mission - in a loaned Brazilian military helicopter - has been hounded by military overflights.
Botero spoke live with the Venezuelan television network Telesur from a jungle clearing where he said the handover was about to occur this afternoon.
He said the group hoped to make it to the lowland provincial capital of Villavicencio by sundown.
Also speaking with Telesur was the guerrilla commander in charge of the handover. He says Colombia's military killed a rebel in his unit in combat this morning.