Rebels control large parts of Ivory Coast
Fresh fighting has reportedly broken out in the west of the Ivory Coast despite continuing discussions on a new power-sharing government for the country.
The claims, from rebel and government officials, come shortly after neighbouring Liberia accused the Ivory Coast of employing Liberian mercenaries to carry out cross-border attacks.
Felix Doh, head of the Ivorian Popular Movement of the Great West (Mpigo), accused the government on Saturday of killing more than 20 civilians in a helicopter gunship attack on the town of Binhouye, close to the Liberian border.
He said that in response to the attack his rebel group would now go on the offensive.
"Once helicopters start bombarding I think the ceasefire is
over," he told French news agency AFP.
"I have given orders to take the offensive."
Ivorian army spokesman Jules Yao Yao denied the allegations, saying that it was the rebels who had been the aggressors, and that his troops had been attacked in the town of Toulepleu.
French troops monitoring the country's shaky ceasefire told BBC correspondent Tom McKinley that they could not confirm either incident as they did not have soldiers in the western area.
However, they did acknowledge that they had seen government helicopters flying to the west on Saturday.
The incident could have serious consequences for the peace process, our correspondent adds.
Three rebel factions now hold Ivory Coast's north, and large swathes of the west, near the border with Liberia.
The Ivory Coast Government, political parties and rebels have tried for weeks to agree on a national unity government called for in a French-brokered peace deal signed in January.
The civil war has killed more than 1,000 people and driven more than a million from their homes.
Spreading to Liberia
Earlier Liberian Defence Minister Daniel Chea said that allowing Liberians to join Ivory Coast troops in an attack was "tantamount to a declaration of war".
President Gbagbo denied links to death squads
About 100 Liberian fighters based in the Ivory Coast captured Toe Town on Liberia's eastern border on Friday, Chea told reporters.
Chea refused to rule out a retaliatory attack by his country, saying: "Anything is possible."
Both sides accuse the other of employing Liberians, and the conflict is now showing signs of spreading back into Liberia.
The Ivory Coast's military has confirmed that some fighting took place, but says it is not certain who the attackers were.
On Saturday, Ivory Coast's President Laurent Gbagbo held a rare press conference to answer accusations that his government is using death squads to assassinate opponents.
Rejecting the idea, President Gbagbo implied that his political opponents and the rebels were the more likely culprits for the murders, saying that an orchestrated campaign was under way to tarnish his name and that of his government.
The death squads have emerged since the current conflict began in September.
A UN report published last month said the death squads appeared to be made up of people close to the government, the presidential guard and a militia from President Gbagbo's ethnic group.