Unidentified gunmen have killed about 20 people in an attack on a cattle market in eastern Burkina Faso, the government said in a statement.There was no immediate claim of responsibility for the attack in the village of Fada N'Gourma, around which the army is carrying out a search operation."Unidentified armed individuals burst into a cattle market in Namoungou village in the region of Fada N'Gourma and attacked the population," the governor, Colonel Saidou Sanou, said in a statement on Friday.