The UN secretary general has urged Ivory Coast's internationally-backed president to investigate hundreds of deaths blamed partly on his supporters.
Ban Ki-moon said he was "concerned and alarmed" about the reports, from the town of Duekoue, but Alassane Ouattara said his followers were not involved.
UN forces are now guarding thousands of civilians taking refuge at a church.
In Abidjan, fighting has continued between troops loyal to Mr Ouattara and those of his rival, Laurent Gbagbo.
Pro-Ouattara forces have beaten back Laurent Gbagbo, to a few key locations, but witnesses say the city is now quieter.
Late on Sunday, officials close to Mr Gbagbo said defence chief General Philippe Mangou had left the residence of South Africa's ambassador, where he took refuge after defecting to Mr Ouattara'#s side last week.
A spokesman told the AFP news agency that Gen Mangou had attended a meeting with Mr Gbagbo; one source told Reuters that he had decided to resume his duties as the commander of Mr Gbagbo troops.
Earlier, UN spokesman Hamadoun Toure told the BBC he had heard gunfire near the presidential palace, currently held by Mr Gbagbo, adding that the situation was very tense.
The airport, which had been secured by UN troops since Friday, is now under the control of French troops, allowing it to re-open.
There were reports that the UN was evacuating around 200 employees from Abidjan.
But the city's pro-Gbagbo TV station called for people to mobilise against what it called a French '"occupation".
Mr Ouattara's forces are reported to be planning a further advance towards the presidential palace and have imposed a curfew on the city.
The violence in Duekoue happened last week, when Mr Outtara's fighters moved south ousting Mr Gbagbo's troops from large swathes of the country. Both sides have said the other was responsible.
Our correspondent says there is evidence of looting in the town and many houses have been burnt down, adding that the situation is much worse than if an army had simply moved through it.
The UN said on Saturday that more than 330 people had been killed as Mr Ouattara's forces took over Duekoue, most of them at the hands of his fighters. However, more than 100 of them were killed by Mr Gbagbo's troops, it added.
"The secretary general expressed particular concern and alarm about reports that pro-Ouattara forces may have killed many civilians in the town of Duekoue," UN spokesman Martin Nesirky said.
"The secretary general said those responsible should be held accountable."
UN officials are still investigating and say the final death toll may be higher. Caritas aid agency estimated that 1,000 may have died.
Mr Ouattara said his supporters were not involved, but told Mr Ban he had ordered an investigation into the killings and would welcome an international inquiry, Mr Nesirky said.
So far Mr Ouattara's supporters say they have found 162 bodies, and that the deaths resulted from fighting between local militias.
France has sent an extra 300 soldiers to Ivory Coast, defence ministry spokesman Thierry Burkhard said, taking the total French force to about 1,400.
The aim of the reinforcement was "to take control over the airport which was also done in co-ordination with the UN mission, to allow the re-opening of this airport to civilian airlines and military flights", he told the BBC.
Mr Burkhard added that the force's mission remained primarily the protection of French nationals, who were being threatened by looters.
"We are currently experiencing in Abidjan a security vacuum because the Ivorian security forces, which until now followed the orders of Mr Gbagbo, answered in great numbers the rallying call made by President Ouattara," he said.
There were no immediate plans to start evacuating foreigners, he said, though about 1,600 were sheltering in a French army camp.
They include about 700 French nationals, some 600 Lebanese citizens and 60 Europeans of assorted nationalities, French media report.