Gunmen who killed seven United Nations peacekeepers, two civilians and at least one soldier in Ivory Coast came from neighbouring Liberia, the Ivorian defence minister said on Saturday.
Paul Koffi Koffi said the raid, on Friday afternoon, highlighted the need for Ivorian troops to carry out cross-border operations in Liberia to improve security.
There was no immediate comment from Liberia but the U.N. said it was reshuffling some of its several thousand troops deployed in the zone as a result of the incident.
The attack highlights simmering tensions and security threats in the west of the world's top cocoa grower despite a year of progress stabilising much of the rest of the country since months of post-election violence last year.
"These people came from the other side of the border. They are militias and mercenaries," Koffi Koffi said, confirming two civilians and "one or two" Ivorian soldiers were killed.
"We must go to the other side of the border to establish a security zone. We will clean up and secure the zone. This will be done, of course, with the agreement of the two countries."
Alassane Ouattara won a 2010 election but only came to power after months of violence that killed thousands after incumbent Laurent Gbagbo refused to concede.
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.
Liberia has denied accusations it is not doing enough to prevent the attacks.
After the incident on Friday, the U.N. Security Council "expressed their deep concern at the prevailing insecurity ... and continued cross-border movements of armed elements, including militias and mercenaries".
Sylvie van den Wildenberg, spokeswoman for the mission known by its acronym UNOCI, said on Saturday that the mission was reinforcing its force in the border area.
"We have several thousand troops in the west, and we are reorganising in light of this incident," she said.
"I can confirm that we are discussing with the authorities ways to increase security and protect civilians in the west. I cannot confirm any plan of a joint operation," she added.