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But the country headed to the brink of civil war after a presidential runoff in early 2011 when then-President Laurent Gbagbo refused to concede defeat after losing to Mr. Ouattara, the internationally recognized winner of the election. Mr. Gbagbo was arrested with the help of U.N. and French forces in April 2011 and now is facing charges of war crimes at The Hague. Mr. Ouattara was sworn in as president soon after.

Following Mr. Gbagbo’s arrest, many of the mercenaries and militiamen who fought for him fled across the porous border into Liberia’s forests, or clandestinely, into its refugee camps.

Western Ivory Coast has remained particularly unstable, and Human Rights Watch said earlier this week that armed groups in Liberia who supported Mr. Gbagbo have killed at least 40 civilians in cross-border raids into Ivory Coast since July. The deaths have all been near Tai, Human Rights Watch said.

“In the four cross-border attacks since June 2011, the motivation appears to have been both political vengeance and related to land conflict — issues that overlap in Ivory Coast’s volatile west. Those killed or whose houses were burned predominantly belong to ethnic groups that largely voted for president Ouattara,” the group said in its report.

Matt Wells, West Africa researcher at Human Rights Watch, told AP that the organization hasn’t confirmed details of the attack but “pro-Gbagbo militants have conducted repeated raids from Liberia into this region of Ivory Coast.”

“The Gbagbo camp often resorted to inciting rhetoric against U.N. personnel during the Ivorian crisis, though today’s deadly attack is the first of its kind during the recent Ivorian crisis,” Mr. Wells said

“Liberian and Ivorian authorities need to quickly work together to bring to account those involved in this heinous act,” he said.

At the end of April, the United Nations said there were about 9,400 peacekeeping troops, 200 military observers and 1,350 international police in the mission along with civilian staff. More than 40 countries are contributing military personnel.

Laura Burke reported from Accra, Ghana. Associated Press writer Edith M. Lederer at the United Nations contributed to this report.