Gunmen attacked Ivory Coast's armed forces in the southwest of the country overnight on Thursday and 15 people were killed, state television said on Saturday.
RTI (Radiodiffusion Television Ivoirienne) said it appeared the attackers had come across the Liberian border into the Tai region of Ivory Coast and caught the soldiers by surprise.
It did not give details on whether the 15 dead were made up of soldiers, attackers or civilians in the area.
Ivorian military spokesman Leon Alla Kouakou confirmed there had been an attack in Tai, but did not give any more details.
West African leaders urged the United Nations and regional grouping ECOWAS in July to step up monitoring of the Liberia-Ivory Coast border after signs mercenaries have been operating there since the end of the Ivory Coast conflict in April.
The leaders of Guinea, Ivory Coast, Liberia, and Sierra Leone said then that insecurity on the Liberia-Ivory Coast border represented a threat for the entire West African region.
"Members of the FRCI (Republican Forces of Ivory Coast) were attacked by surprise. The toll from the attack was 15 dead," RTI said, adding reinforcements were being sent to the porous border region, which is mostly dense forest.
The government has said Liberian mercenaries were hired by supporters of former Ivorian President Laurent Gbagbo during the four-month post-election conflict which ended with his capture and arrest in April.
FRCI was the name given to the body of fighters that battled to remove Gbagbo from power and allow Alassane Ouattara, widely believed to have won last November's presidential election, to take power in the country which is the world's largest cocoa grower.