FERGUSON, United States (AFP) - Police arrested 15 people in the protest-hit St Louis suburb of Ferguson on Friday in more demonstrations demanding justice for an unarmed black teenager killed by police.

There have also been sporadic demonstrations around the United States after a Missouri grand jury decided earlier this week not to indict the white policeman who shot dead 18-year-old Michael Brown in August.

Monday's announcement that officer Darren Wilson will not face charges sparked protests, looting and eruptions of gun fire in Ferguson, and demonstrators were again on the streets in the predominantly black suburb on Friday.

"Police in Ferguson issued warning to protesters leave streets. Some protesters ignored warnings. Were taken into custody," St Louis County Police Department tweeted.

In a subsequent tweet, it added: "Confirmed: 15 arrested in Ferguson; one facing assault on a law enforcement officer charge. Other protesters remain peaceful."