Attackers wielding machetes and guns killed more than 1,000 civilians in the neighbourhood of an Ivory Coast town controlled by forces fighting to instal the internationally-recognised president, a charity said.

The United Nations mission in Ivory Coast said a team was investigating the alleged mass killings in western Duekoue. It said most of the nearly 1,000 peacekeepers based there were protecting about 15,000 refugees at a Catholic mission in the town at the time.

Patrick Nicholson of the Roman Catholic charity Caritas said workers visited Duekoue on Wednesday and found hundreds of bodies of civilians killed by bullets from small-arms fire and hacked to death with machetes.

They estimated more than 1,000 civilians were killed, he said.

The International Federation of the Red Cross put the death toll at about 800, in separate and independent visits on Thursday and Friday.

Mr Nicholson said the killings occurred over three days in a neighbourhood controlled by fighters loyal to internationally-recognised President Alassane Ouattara, though it was not clear who the perpetrators were.

"The massacre took place in the Carrefour quarter of town, controlled by pro-Ouattara forces, during clashes on Sunday 27 March to Tuesday 29 March," Mr Nicholson said.

"Caritas does not know who was responsible for the killing, but says a proper investigation must take place to establish the truth."

He said the victims included many refugees from fighting elsewhere in the country, where rival forces had been battling over a disputed November election.

Caritas' investigation would indicate that people were killed at close quarters in a small neighbourhood of a town of just 50,000 people as pro-Ouattara fighters began a two-pronged assault that brought them swiftly to Abidjan, the commercial capital and seat of power, within days.

The charges would be a strong blow to the embattled government of Mr Ouattara, who is calling for entrenched incumbent Laurent Gbagbo to cede power after losing November's poll.

Mr Ouattara's camp denied forces fighting for it were involved in any atrocities including in western Ivory Coast, but did not refer to the latest allegations.

Previously, the United Nations put the death toll at 492 from four months of fighting.

United Nations military spokesman Colonel Chaib Rais said he had "no special report" of mass killings.

"There was fighting two days before, on Sunday, and people were killed, but I cannot confirm those numbers," he said.

On Monday, fighters loyal to Mr Ouattara said they seized Duekoue from Gbagbo forces. But Mr Nicholson said interviews with survivors indicated pro-Ouattara forces had control of the Carrefour neighbourhood from Sunday.

ICRC spokeswoman Dorothea Krimitsas said "inter-communal violence" erupted there, apparently on Tuesday.

"We think there is a risk that this kind of event can happen again and hope that by calling today again for protection for the civilian population, we hope that such events can be avoided in the future," she said from Geneva, Switzerland.

The area has been a hotbed for conflict between two rival tribes supporting Mr Ouattara and Gbagbo, who refuses to accept his election defeat.

The International Organisation of Migration said on Friday that tens of thousands of refugees had overcrowded Duekoue and that others who had fled the violence there "are now stranded along the route, in fear for their lives".

It said some of those slaughtered were apparently killed by "mercenaries" from nearby Liberia. Liberian mercenaries have been reported to be fighting for both Gbagbo and Mr Ouattara.

Mr Ouattara's government, in a general statement on Friday responding to allegations of abuses by Amnesty International, blamed any killings on Gbagbo forces acting as they retreated.

Mr Ouattara has long tried to distance himself from the northern-based fighters taking up his cause who fought in a brief civil war almost a decade ago that left the country split in two. Those fighters were accused of many atrocities at the time.

But his repeated calls for an international military intervention to force out Gbagbo and end the violence have gone unheeded. This week he appeared to change tack as the rebel fighters began a swift advance on Abidjan, calling them the "Republican Forces".

"The government firmly rejects these accusations and denies all implication of the Republican Forces of Ivory Coast in any possible violations," said Friday's statement.

Human Rights Watch said it had documented abuses, with the vast majority perpetrated by forces loyal to Gbagbo against real or perceived Ouattara supporters, as well as against West African immigrants and Muslims.

But the New York-based organisation said atrocities committed by pro-Ouattara forces risked amounting to war crimes, including three detainees burned alive and four whose throats were slit, all in Abidjan.

"Human Rights Watch has also received credible reports of abuses committed when Ouattara's forces took control of several towns in western (Ivory Coast)," it said.

"The killing of civilians by pro-Ouattara forces, at times with apparent ethnic or political motivation, also risks becoming a crime against humanity should it become widespread or systematic."