FERGUSON, United States (AFP) - Police fired tear gas to break up protesters in another night of unrest in this Missouri town where a black teen was shot and killed by a white police officer nine days ago.

A street-wide line of police in riot gear, supported by an armoured SWAT vehicle and a helicopter overhead, repeatedly ordered the crowd of about 100 to disperse on Monday night..

They then let loose with a volley of tear gas just after 11 pm (0400 GMT), sending the crowd - smaller than one that clashed with police on Sunday - scurrying into the darkness.

At least two people were seen being arrested, and gun shots - described by one senior police officer as "potshots in the area" - were heard.

But National Guard troops who deployed earlier on Monday in Ferguson to reinforce police remained out of sight, taking up a position instead at a police command centre.

Monday night's standoff occurred near the residential street where 18-year-old Michael Brown was fatally shot by a local police officer in this St Louis suburb.

Pathologists retained by Brown's family said on Monday that the student had been shot at least six times, including twice in the head.

His death has triggered daily protests and nightly clashes with law enforcement officials, and prompted Missouri Governor Jay Nixon to call out the National Guard.