Rival youth gangs have subjected residents and shopkeepers in a north London street to a six-month reign of terror, believed to be fuelled by two feuding families.

Traders in Salcombe Gardens, Mill Hill, say the road is blighted by antisocial behaviour from groups of up to 40 teenagers who gather there late into the night.

Trouble escalated on Tuesday when a 19-year-old man was violently assaulted by youths. There are now fears the next incident could be fatal, following reports that the teenagers are carrying knives.

One shopkeeper, who did not wish to be named, said: "There were about 20 of them and they kicked around this kid and then all ran away. My customer said she saw a knife."

Although police and an ambulance crew attended the scene shortly after 5pm, the victim, who suffered facial injuries, declined treatment and refused to speak to the officers.

A police spokesman said an atmosphere of fear was preventing residents and shopkeepers from coming forward.

He said: "They go to other police stations and give them bits of information, but they are scared.

"The Safer Neighbourhoods team has redirected its patrols to spend more time in the area, and is trying to get people to talk."

He added that a feud between two local families was thought to be at the centre of the problem.

A 24-year-old employee at one of the shops, who asked not to be named, believes the police are powerless to act.

He said: "The teenagers steal things all the time. The police come, take our complaints and then leave. The teenagers know the police can't do anything."

The youths gather by the Mill Hill East Church.

Church leader Ray Ashley, who lives in the road, said: "This is actually the third time I've heard reports of a knife. I'm very concerned that something might happen."

Barnet Homes, which manages and maintains council accommodation, in conjunction with the police, have installed a 'mosquito box' in the street, which emits a loud, high-pitched sound. It is audible only to young people and is intended to deter them from particular areas.

The police spokesman admitted that the device does not seem to have had an impact.