Missouri state police will take charge of security in the town of Ferguson, the state governor said today, after local officers shot dead an unarmed teenager, triggering four days of violence.

Earlier, US President Barack Obama called for peace on the streets of the town and urged authorities to be transparent in their investigation.

Michael Brown,18, was shot dead in the mostly black suburb of Ferguson, Missouri on Saturday after what police said was a struggle with a gun in a police car.

A witness in the case told local media that Mr Brown had raised his arms to police to show that he was unarmed before being killed.

Protesters have gathered in the St Louis suburb every night since tense standoffs with heavily armed police.

"Lately it looks like a war zone and that is not acceptable," Governor Jay Nixon said of Ferguson, outside St Louis.

The county police have been criticised for their military-style crackdown on protests.

"We will all need to join hands to rebuild the trust that's been lost and help this community regain its stability," Mr Nixon said, appointing a local African-American officer, Captain Ron Johnson, to take charge.

Police have been slow to release information about the shooting, except to say that it followed a struggle between the unnamed officer and Mr Brown.            

Details about the incident are in dispute. A witness who was walking with Mr Brown at the time has said in media interviews that Mr Brown put his hands in the air and was not struggling with the officer.

He said the officer fired multiple times into Mr Brown's head and chest.

The witness, Dorian Johnson, was expected to meet prosecutors and investigators yesterday, local media reported.

His lawyer, Freeman Bosley, a former St Louis mayor, did not immediately answer requests for comment.

Police have declined to release the name of the officer involved in the incident, citing concerns for his safety, a decision that has been criticised by demonstrators who have asked for more transparency.

The officer has been placed on administrative leave.

Mr Johnson, who takes charge of security, said police would show respect to the citizens as they try to restore calm.

"I grew up here and this is currently my community and my home. It means a lot to me personally that we break this cycle of violence and build trust, and show the utmost respect," he said.

"I understand the anger and fear that the citizens of Ferguson are feeling, and our police officers will respect both of those," he added.