US officials have released dashboard camera video showing a white police officer fatally shooting a black teenager 16 times in Chicago last year.

Officer Jason Van Dyke has been charged with first-degree murder for killing Laquan McDonald, 17.

Police say he refused to drop a knife.

Van Dyke said through his lawyer and police union he had feared for his life.
Chicago Mayor Rahm Emanuel has appealed for calm in the city.

Police use of excessive force against African Americans has been the subject of nationwide protests for months.

Mayor Emanuel told a news conference it is fine to be passionate but it essential to remain peaceful.