The shelling of an Abidjan market by Ivory Coast security forces which killed at least 25 people may be a crime against humanity, the UN says.
Allies of disputed President Laurent Gbagbo have denied UN claims they fired the shells.
They landed in the district of Abobo, which is under the control of militias who back his rival, Alassane Ouattara.
Mr Gbagbo refuses to step down although Mr Ouattara is widely recognised as the winner of last year's poll.
A statement from the UN mission in Ivory Coast says that about 100 people were killed or maimed by at least six 81mm mortar shells.
"Such an act, perpetrated against civilians, could constitute a crime against humanity," it says.
But Ahoua Don Mello, a spokesman for Mr Gbagbo's government, told the AFP news agency the accusations against the security forces were part of a "conspiracy" between the UN, Mr Ouattara's supporters and former colonial power France to oust Mr Gbagbo.
Mr Mello later read out a statement on national TV, saying Mr Gbagbo was open to talks as part of an African Union peace plan.
He also accused regional powerhouse Nigeria of transporting 500 mercenaries to pro-Ouattara forces based in the northern town of Bouake.
Pro-Gbagbo forces have been accused of firing at peaceful demonstrators on several occasions recently.
Some 370,000 people have fled recent clashes in Ivory Coast's main city, Abidjan - many from the Abobo district and more were leaving their homes on Friday.
International sanctions have been imposed on Mr Gbagbo and scores of his allies in a bid to force him from power.
France wants the sanctions tightened, reports the Reuters news agency.
"Given what is happening and the rising number of violent acts it is important we send a strong signal to reinforce the sanctions regime in place," a foreign ministry official said.
The UN says more than 400 people have been killed since the November election.
The African Union has given Mr Gbagbo until 24 March to organise a handover of power but he shows no signs of stepping down.
The election was supposed to reunify the country, which has been divided since a 2002-3 conflict.
Pro-Ouattara forces control the north of the country and many fear that a civil war could resume.
The New Forces rebels have mainly stayed on their side of the ceasefire line but they have seized some ground in the west.
Fighting here has led some 75,000 to flee into neighbouring Liberia.
Ivory Coast used to enjoy the highest living standards in West Africa and the world's biggest cocoa producer was seen as a haven of stability in the troubled region.