OUAGADOUGOU, June 23 (Reuters) - Four police officers who had gone missing after a militant attack in Burkina Faso on Monday have been found dead, raising the police death toll to 15, police sources said on Wednesday. The ambush occurred while the unit was on a relief mission to the town of Yirgou, which has been hit by recent violence, Security Minister Ousseini Compaore said in a statement. read more Initially 11 police officers were reported killed and four missing. Five police sources told Reuters that the four missing were now confirmed dead. Attacks by militants linked to al Qaeda and Islamic State are surging across Africa"s Sahel region, killing thousands and displacing millions across Burkina Faso, Mali and Niger. About 1.2 million people have been displaced by the violence in Burkina Faso. At least 132 people were killed by insurgents last month in the worst single attack yet. read more