Chicago's mayor has appealed for calm after a "chilling" video was released of a white policeman shooting dead a black teenager, Sky News reported Wednesday.

Laquan McDonald was shot 16 times on 20 October 2014 while running down a street in the city.

Police say the 17-year-old refused to drop a knife which was recovered from the scene and Chicago police officer Jason Van Dyke has been charged with first-degree murder.

Van Dyke was the only officer present at the scene who opened fire. Prosecutors say the officer emptied his 9mm pistol of all 16 rounds and that he was on the scene for just 30 seconds before he started shooting.