The attack occurred near the town of Tai, close to the Liberian border, said Josephine Guerrero, spokeswoman for U.N. peacekeeping operations at the world body's New York headquarters.
She gave no other details.
[Updated at 4:45 p.m.: U.N. Secretary General Ban Ki-moon later condemned the attack "in the strongest possible terms" and appealed to the Ivory Coast government to "do its utmost to identify the perpetrators and hold them accountable for this wanton attack."
There were reports of an unspecified number of Ivory Coast citizens killed in the same ambush, Ban said in his statement, adding that he "remains seriously concerned about the continued instability in the border areas" in the post-election crisis conditions.]
The U.N. mission was launched in 2004 in the midst of the Ivory Coast's civil war and has continued because of violence in the aftermath of the 2010 presidential election. The fighting pitted followers of the defeated president, Laurent Gbagbo, and those of Alassane Ouattara, the challenger who won the vote.
Gbagbo had refused to cede power, and was finally forced from office by French and U.N. troops.
The U.N. peacekeepers killed Friday were operating in an area where ethnic groups loyal to Ouattara have come under attack from Gbagbo loyalists. Human Rights Watch reported this month that cross-border raids have killed about 40 people, including women and children, in the last year.
The United Nations has about 11,000 peacekeeping troops, military observers and international police in Ivory Coast. The 8-year-old mission involves forces from 40 countries.
According to the U.N. website, at least 30 other peacekeepers have been killed this year while on duty with world body missions. The operations suffered 109 fatalities last year, and the death toll since foreign missions began in 1948 now stands at 2,923.
-- Carol J. Williams in Los Angeles
Photo: U.N. peacekeepers, in the background, keep an eye on a fuel station in Abidjan, Ivory Coast, in March 2011. Credit: Associated Press