Navi Pillay, a former UN war crimes judge, also said that deteriorating security conditions in the West African country prevented investigations of atrocities, including at least two mass graves.
"The international criminal justice system that has developed over the past 15 or so years has given us a tool of accountability we did not have before," Ms Pillay said.
"No longer can heads of state, and other actors, be sure that they can commit atrocious violations and get away with it."
The United Nations had been invited by all parties to certify the results of the 28 November presidential run-off vote. The UN declared Alassane Ouattara the winner, endorsing the announcement by the Ivorian electoral commission. But Mr Gbagbo has refused to step aside now for more than a month.
West African leaders have said they are prepared to use military force to push Mr Gbagbo out, but are giving negotiations more time at present.
Ivory Coast was divided into a rebel-controlled north and a loyalist south by a 2002-2003 civil war, and the long-delayed presidential election was intended to help reunify the nation. However, tensions over the outcome have sparked violence including several attacks on UN peacekeepers.