OUAGADOUGOU( Reuters)-Unidentified assailants killed seven police officers and wounded five more in northern Burkina Faso on Friday morning, the government said in a statement.... Islamist militant groups with links to al Qaeda and Islamic State are active in the area, which borders Niger and Mali.... Search operations are underway to track down the assailants, the statement said. Five police officers were killed in a similar attack two weeks ago.( Reporting by Thiam Ndiaga; Writing by Nellie