PARIS (AFP) - France's black Justice Minister Christiane Taubira on Tuesday waded into the conflict over racially charged killings in the United States, quoting Rastafarian reggae icon Bob Marley on Twitter to express her anger.

"Kill them before they grow", the minister tweeted, citing Marley who sang the phrase in his 1973 hit song "I shot the sheriff".

Taubira's tweet came as riots erupted in the suburb of Ferguson outside St Louis after a grand jury chose not to press charges against a white officer who shot dead black teen Michael Brown in what he said was self-defence.

The grand jury decision further fuelled racial tensions in the United States after Cleveland police shot dead 12-year-old Tamir Rice, who was waving around what turned out to be a toy gun on a playground.

"How old was Mickael (sic) Brown? 18. Trayvon Martin? 17. Tamir Rice? 12. How old next? 12 month? 'Kill them before they grow' Bob Marley", Taubira tweeted in English.

An unarmed Martin was killed in 2012 by a neighbourhood watch volunteer during what he described as an altercation at night in the street.

Taubira later explained to France Info radio that she had wanted to express solidarity "in an extremely painful situation." "I am not making a value judgement on United States institutions (but) when the feeling of frustration is so strong, so deep, so lasting and massive, you have to ask yourself about the trust in these institutions," said the minister.

"One realises this only happens to the same people, African-American kids. So there is the problem of a certain number of cliches, portrayals, prejudices which can create terrible reflexes."