Ivorian government forces vowed to hunt down those responsible for an ambush that killed at least seven United Nations peacekeepers as Liberia sealed its border amid fears that the gunmen had used the country to stage the attack.
Liberian information minister Lewis Brown said the president had ordered the immediate deployment of the armed forces to the border in the wake of Friday's attack near the Ivorian town of Tai, which also left at least eight civilians dead and sent hundreds of people fleeing the area.
Human Rights Watch has said that militants have staged several cross-border attacks from Liberia, leaving at least 40 people dead in the last year.
The violence has been blamed on mercenaries and militiamen who fought for ex-Ivorian president Laurent Gbagbo and then fled across the porous border into Liberia's forests, following his arrest last year.
In a report on Wednesday, the human rights group had accused the Liberian government of failing to respond to the presence of armed groups on the border or to the recruitment of child soldiers.
Liberian defence minister Brownie Samukai said efforts were being made to prevent the use of Liberia's territory as a launching ground for attacks. "We all are surprised by the deterioration of the situation in Tai," he said, adding that it was difficult to establish the nationalities of those involved.
Ivory Coast was brought to the brink of civil war when Gbagbo refused to cede power after losing a November 2010 election. The United Nations estimates at least 3,000 people were killed in the six months of violence that followed, and more than 60,000 Ivorian refugees remain in neighbouring Liberia.
Gbagbo was arrested with the help of UN and French forces in April 2011, and is now facing charges of war crimes at The Hague.
Friday's attack marked an unprecedented assault on UN peacekeepers, who have been in the country since 2004. "This is the first time we have ever had such a type of attack in Ivory Coast. It's a very tough time for the whole mission here right now," said Sylvie van den Wildenberg, acting spokeswoman for the UN mission in Ivory Coast.
Ivory Coast's deputy defence minister Paul Koffi Koffi said government forces, along with Liberian and UN forces, would launch an operation on June 15 to find the men responsible, whom he described as "militiamen or mercenaries".