Rapper 50 Cent has warned protesters demanding action over two police killings in the U.S. that it is "not fair" to speak out without knowing all the facts, insisting there is often a "grey area" in grand jury cases.

The In Da Club hitmaker, real name Curtis Jackson Iii, had several brushes with the law before finding fame and was sentenced to three to nine years in prison for drugs and gun possession.

He says his dealings with law enforcement authorities made him realise there is often "a defence" to their actions on the streets and a "balance" in every incident, and he has vowed not to make a judgement in the controversial cases of Michael Brown and Eric Garner without hearing all the facts.

Demonstrators have taken to the streets in several major U.S. cities following recent separate grand jury rulings not to indict police officers involved in the shooting of teenage thief Brown in Missouri and the death of Garner, who was apparently placed in a chokehold as he was arrested in New York for illegally selling cigarettes.

Jackson tells the New York Daily News, "I think it's interesting the timing of everything that's happening. I've stayed pretty quiet about how I felt about it personally, because I don't have all the information in those situations. I've had people make decisions about me without knowing how I felt about things. It's not really fair to say it if you don't know.

"I grew up in an environment where a lot of people, I know for a fact, have criminal intentions, so it would raise a defense on one end and raises the aggression level on the other at the same time, so you see there's a balance. There's right and wrong, a lot of times you look at things and you kind of see things in a gray area."