The UN human rights office Tuesday said at least 536 people had been killed in west Ivory Coast since the end of March, the majority in the town of Duekoue, and warned the toll could be higher.
"Our investigation team in the west has been reinforced... we have so far established that 536 people were killed in the west of the country," Ravina Shamdasani, spokeswoman for the office of the UN High Commissioner for Human Rights said.
She said the killings were likely to have been committed "since the end of March" and that bodies were found in several towns including Duekoue, Guiglo and Blolequin.
Shamdasani warned that the final toll could be higher and called for accountability, saying that it is "necessary for national reconciliation."
The UN on Friday said its human rights investigators had found more than 100 bodies in 24 hours in the same areas of western Ivory Coast, some in mass graves after what appeared to have been ethnically driven killings.
That discovery followed an earlier count of 229 corpses previously found in Duekoue and the UN human rights office said the violence was escalating.
On Saturday, the campaign group Human Rights Watch accused forces loyal to President Alassane Ouattara of killing or raping hundreds of people and burning villages during a rampage in late March.
The rights group revealed new evidence of summary killings of supporters of arrested strongman Laurent Gbagbo in the far west as Ouattara followers seized Gbagbo territory.
The UN has been more cautious about assigning responsibility for incidents, underlining that the recent national power struggle had grafted on to ethnic tensions in the region.
The International Committee of the Red Cross has already said up to 800 people were killed during fighting for the control of Duekoue in late March.
Relief agencies warned that Monday's arrest of Gbagbo did not spell the end of the humanitarian crisis in the west African country.
"At the moment, the capture of Laurent Gbagbo does not change the humanitarian situation," said Elisabeth Byrs, spokeswoman for the UN Office for the Coordination of Humanitarian Affairs.
"We need to have access to Abidjan to help people in this city," she said, stressing that urgent aid was necessary. The International Organization for Migration spokesman Jumbe Omari Jumbe stressed that "humanitarian assistance will be needed for many many years to come."