It was not immediately clear who attacked the UN troops or if any Ivorian troops they were patrolling with were harmed.
The United Nations said the mission had only recently increased its presence in the area, near the towns of Para and Tai, to boost efforts to protect civilians. The troops came under attack just a few kilometres from the border with Liberia.
“According to the provisional death toll, seven blue helmets were killed in an ambush … seven blue helmets from Niger,” said Sylvie van den Wildenberg, spokeswoman for the mission known by its acronym UNOCI.
UN Secretary-General Ban Ki-moon told reporters in New York that he was “saddened and outraged” by the attack.
“I understand that their colleagues 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,” he said.
The world’s top cocoa grower is gradually recovering from months of violence last year that followed a 2010 election.
The vote was won by Alassane Ouattara but incumbent Laurent Gbagbo refused to cede until defeated by French and UN-backed local forces who supported Ouattara.
New York-based Human Rights Watch warned earlier this week that Liberian mercenaries and Ivorian fighters who fought on behalf of Gbagbo in the brief civil war last year were launching attacks on Ivory Coast from Liberia.
The rights campaigner said the combatants behind the raids, which have killed 40 people since last July, are receiving support from individuals in the region.
Authorities in Togo arrested Moise Lida Kouassi, a close advisor to Gbagbo, on Wednesday. Ivory Coast accused him of plotting to destabilise the government from exile.
Ivory Coast Defence Minister Paul Koffi Koffi told Reuters that Ivorian troops were preparing operations with Liberian forces, and peacekeepers from the UN missions in the two countries to eliminate threats emanating from Liberia.
“This is a real problem for us, and we are going to go to Liberia in order to go after these militias or mercenaries who live there and who are destabilising our western border,” he said, adding that the operations would begin on June 15.
UN officials could not immediately confirm that the military operations were going ahead.