Seven UN peacekeepers from Niger were killed in an ambush in western Ivory Coast Friday, in the deadliest attack on the force since its deployment in 2004, a UN spokesman said.
The UN Security Council swiftly "condemned in the strongest terms" the deadly ambush.
The 15-member council called on the Ivory Coast government to "work with all relevant parties to identify and bring the perpetrators to justice."
UN leader Ban Ki-moon said he was "outraged" by the killings of the peacekeepers and warned that more UN troops "are still in danger".
"Even tonight, after the attack, more than 40 peacekeepers remain with the villagers in this remote region to protect them from this armed group," the UN chief said. "My thoughts are with these brave peacekeepers and the community they are protecting."
He called on the Ivory Coast government "to do its utmost to identify the perpetrators and hold them accountable for this deadly attack".
The country's deputy defence minister Paul Koffi Koffi said the attackers had crossed over from neighbouring Liberia, adding that two Ivorian soldiers and at least one civilian may also have been killed.
The UN denounces the "very serious violation of international law", a spokesman for the United Nations Operation in Ivory Coast (UNOCI) added.
The peacekeepers had been patrolling in an area between two villages after hearing rumours of an imminent attack on communities in the region.
"There's panic in the villages, many are fleeing into the forest, others are heading for Liberia," a resident of Para village told AFP by phone.
The mayor of nearby Tai village, Desire Gnonkonte, confirmed that residents were fleeing.
Ivory Coast's west is by far the most unstable part of the country and has been plagued by deadly attacks since a political and military crisis that started at the end of 2010 and left some 3,000 people dead throughout the country.
In a report published Wednesday, Human Rights Watch said at least 40 people had been killed since July 2011 in raids the group blamed on fighters loyal to Ivory Coast's ex-president Laurent Gbagbo.
Gbagbo was captured on April 11, 2011 and has been in custody in The Hague since November on allegations of crimes against humanity.
Koffi said that some 50 attackers had crossed the river that marks the border with Liberia before descending on the villages of Saho, Para and Nigre.
"We think these are the same groups who have been responsible for all the attacks in the area in recent months," he said.
The Special Representative of the United Nations Secretary-General for Ivory Coast, Bert Koenders, condemned the unprecedented ambush against the UNOCI troops.
The seven "were part of a patrol that was on a mission south of the locality of Tai, in a zone where UNOCI recently strengthened its presence due to threats of attacks against the civilian population", he said in a statement.
UNOCI was first deployed to the west African country in 2004 and currently counts more than 10,000 uniformed personnel.