Rebels in Ivory Coast say the peace process is now dead and buried after their positions were attacked.
Western Ivory Coast is awash with rival ethnic militias
United Nations peacekeepers arrested 68 people and seized weapons in the western, cocoa-producing region.
Rebels said the attackers belonged to a pro-government militia. This was denied by a presidential aide who, however, also cast doubts on mediation efforts.
Some 10,000 peacekeepers are patrolling a buffer zone between the rebel-held north and the loyalist south.
Rebel New Forces spokesman Sidiki Konate said the militiamen had crossed the buffer zone in order to attack their positions.
"By these acts of war, [President] Laurent Gbagbo has definitively buried all the mediation efforts of the African Union and the international community," he said.
"The New Forces' armed forces decree a maximum alert in all their zones and are currently carrying out systematic searches."
Another rebel commander said that 32 people had been killed in the fighting - 30 attackers and two rebels - but this has not been independently verified.
But presidential adviser Etienne Tahizoue told the BBC's Network Africa programme that the attack had been carried out by local villagers without any official backing.
"That is nonsense," he said.
"The villagers are tired. They don't want the rebels around their area."
But he also cast doubts on the viability of mediation attempts.
"We've given the peace efforts two-and-a-half years. It's time now to take our defence ourselves," said Mr Tahizoue, who is from the area where the fighting took place.
These are the first clashes since last November, when loyalist forces bombed rebel positions in their Bouake headquarters.
Earlier Felix Maho, an official with the FLGO militia, a pro-government militia said the group had clashed with rebels in the town of Logouale.
Logouale is in Ivory Coast's cocoa-producing "Wild West", which has seen some of the fiercest clashes during the conflict.
Fighting there also spilled over the border into Liberia, which has a similar ethnic make-up to western Ivory Coast.
Ivory Coast is the world's largest producer of cocoa, the raw ingredient for chocolate.
Last month, the UN expressed its concern about mounting tension in Ivory Coast and called for swift disarmament.