Seven UN peacekeepers from Niger have been killed in an ambush in southwestern Ivory Coast, the UN has said.
The attack took place near the border with Liberia, UN officials say.
UN Secretary-General Ban Ki-Moon said he was “saddened and outraged” by the deaths.
UN peacekeepers were deployed to Ivory Coast in 2004 to help end the country’s civil war and have stayed through the country’s recent political crisis.
“These brave soldiers died in the service of peace,” Mr Ban said.
Mr Ban offered his “deepest condolences” to the government of Niger and urged the Ivorian government to “identify the perpretators and hold them accountable”.
It is the first attack of its kind in the country, a UN spokesman told Agence-France Presse (AFP).
The peacekeepers were patrolling in an area between the villages of Tai and Para after hearing rumours of an imminent attack on communities in the region, UN officials said.
“Even tonight, after the attack, more than 40 peacekeepers remain with villagers in this remote region to protect them from this armed group,” Mr Ban said.
“We are moving in reinforcements as soon as we have daylight,” Kieran Dwyer, the spokesman for the UN peacekeeping department, told the Associated Press.
The area has seen a number of cross-border attacks over recent months, blamed on Liberian mercenaries and Ivorian militia fighters who fled Ivory Coast following the arrest of the former president Laurent Gbagbo last year, the BBC’s John James in Abidjan reports.
Just last week, UN reinforcements were sent to the region following rumours of a planned attack, our correspondent adds.
Earlier this week, campaign group Human Rights Watch published a report saying militants loyal to Mr Gbagbo based in Liberia had killed at least 40 people in cross-border raids since last July.