More than 100 pro-government militants waged a pre-dawn attack near Logouale, some 450km northwest of Abidjan.
The attack means troops crossed the confidence zone dividing rebel north from government south that is patrolled by some 10,000 UN and French peacekeepers, deputy rebel spokesman Antoine Beugre said.
It was the first major ceasefire violation since government planes bombarded rebel positions for three days in November, killing a reported 85 civilians according to a rebel toll.
"This attack is the umpteenth violation of the ceasefire by troops of [President] Laurent Gbagbo after the events in November," rebel spokesman Sidiki Konate said in a statement.
"With these acts of war, President Gbagbo has definitively buried all the efforts at mediation by the African Union and the international community," Konate added, and urged all civilians and rebel fighters to be on maximum alert.
An unknown group calling itself the Ivorian Movement for the Liberation of the West of Ivory Coast (MILOCI) claimed responsibility for the early-morning attack, which ended with some 70 militants in custody.
"We attacked the rebel positions to control the town," said Pasteur Gammi, the self-proclaimed leader of MILOCI, one of the dozens of pro-government militias operating with the tacit or overt support of the Gbagbo administration in the area on the border with Liberia.
"We think that the UN operation in Ivory Coast and the French military here are not doing their jobs, so we decided to go and disarm the rebels ourselves so as to ease the suffering of our families, who live in the occupied zone."
UN officials said they would
expand their presence in the area
Humanitarian and diplomatic officials in the area said, however, that the militia was very likely a front for the Front to Liberate the Great West (FLGO) movement, based in the government stronghold of Guiglo, or for the Ivorian national army itself.
Details emerging from the isolated zone of the world's top cocoa producer were sketchy, as both UN peacekeepers and the rebels themselves said they had apprehended the militants, all of whom were carrying weapons such as AK-47 rifles.
There were no immediate reports of casualties, though a UN official said the attack would mean that the peacekeeping presence in the area would be increased and expanded to extend beyond the perimeter of the confidence zone, which has since September 2002 cut a 400km swathe across the centre of the country.
Once an anchor of stability and economic prosperity for a troubled region, Ivory Coast has tumbled into chaos since a September 2002 coup sparked a low-level civil war.
Exhaustive French and West African mediation efforts have been virtually for naught, as a 2003 peace pact has yet to be fully implemented and the rebels still hold fast to their arms.