Residents in Melbourne's west have been urged by police not to form "neighbourhood patrols" amid growing community concern about the increase in violent burglaries and carjackings across the city.

Key points: Confronting offenders not worth the risk, police warn

Confronting offenders not worth the risk, police warn Police trying to better understand motives behind crime 'phenomenon'

Police trying to better understand motives behind crime 'phenomenon' Victorians urged to lock cars, homes, take extra security measures

The patrols came to the attention of police when the issue was raised at community meetings in Hillside and Derrimut.

On the Brimbank Eyewatch Facebook page, a forum run by police to communicate with people about local issues, police discouraged the patrols, saying they could not "stress enough the risks that go with it".

Assistant Commissioner Stephen Leane said he understood why people felt scared but warned against confronting offenders.

"I appreciate people are frightened by this and they want to protect themselves, their family and their homes," he said.

"You can reasonably defend yourself. But self-defence does not give you any right to walk around the street armed with baseball bats, or to walk around your front yard to ward off anybody you think might be coming to steal your property.

"They may not be, they may be just walking past. It doesn't help and it will escalate the tension in the community."

Assistant Commissioner Leane said residents could never be sure what an offender was capable of doing.

"My caution around that is simply: if it's about property and you're trying to protect personal possessions, they are replaceable," he said.

"Is it worth potentially being seriously injured, or worst-case scenario killed, as a result of trying to protect a set of wheels in your driveway that you can replace?"

'Scared residents' want to 'stand up' to offenders

The concern from residents was in response to the dramatic increase in the number of carjackings and robberies across Melbourne, but particularly in the south-west and south-east suburbs.

On Tuesday 13 teenagers were arrested over one single burglary at a home in St Albans.

One man on the Eyewatch forum asked whether it was illegal to carry a baseball bat while driving "in case you get boxed in and cannot escape", possibly referring to the tactic used by offenders to nudge a car while on the road, drawing the driver out to inspect the damage before stealing it.

Another wrote that some "scared residents" were bound to take matters into their own hands in retaliation.

"You aren't going to stop the citizens patrolling the area because the offenders have no respect for the law and once the court sets them free they will be straight back doing it again," he wrote.

"So this is where old school justice is going to take over and people are going to do what they have to to feel safe and stand up to these kids and teach them the respect that they need."

Motivation behind burglaries unclear, police say

Assistant Commissioner Leane said most of the thefts were targeting cars, and they were throwing all their resources at both prosecution and prevention.

But he said it was still unclear the exact motives behind the rise in crime, which he labelled a "phenomenon".

Assistant Commissioner Stephen Leane from Victoria Police. ( ABC News: James Dean )

"You can't steal most cars now without having the keys, so to get the keys we've seen an escalation [in] behaviours from particularly a young cohort, mostly young ... who are getting access to keys in any way they can," he said.

"And they're now resorting to confrontation, direct confrontation, whether it's a carjacking or a home invasion."

He said police were holding a youth summit involving about 200 people from around the country to try and better understand why the thefts were taking place.

But Assistant Commissioner Leane dismissed the idea the crimes were mostly being committed by African youths.

"I think they're very visible, and I think they are very visible for many of us because we don't have many South Sudanese friends that we're used to being around with," he said.

"It is, I've used the words before, the united nations of offending. We're seeing a cross-group of offenders who are stealing cars in this way."

Three months ago police launched a special operation in response to the increase in carjackings and theft, increasing resources and patrols in areas at most risk.

Almost 80 people have been arrested in that period.