A senior presidential source said on Thursday that unidentified people armed with guns and machetes had attacked local villagers in Siegouekou, which is close to the western cocoa town of Gagnoa.
"Unidentified people smashed down doors with guns and knives and cut the throats of some and fired point blank at others," he said.
The villagers retaliated by attacking farmers from other parts of Ivory Coast or from neighbouring West African countries living in the area.
A military source said he knew of 16 dead and believed two more people had died of their injuries after the clashes.
Clashes between rival ethnic groups over precious cocoa land are common in the area, where a civil war that erupted in 2002 has inflamed tribal rivalries.
Hundreds of migrant farmers, who form the backbone of the cocoa workforce in the world's top cocoa producer, have been chased off their plantations by villagers who say the land is theirs.
The presidential adviser said it was not immediately clear who the attackers were but said they had left a note saying they would attack again unless they got their land back.