Seven people have been arrested during a two-week crackdown on cannabis factories.
Bryan Dent, West Yorkshire Police's force drugs co-ordinator, said Vietnamese gangs had become involved in cultivating cannabis from dens in West Yorkshire, mostly in Bradford.
He said seven Vietnamese suspects had been arrested, all of them in Bradford.
Some had been detained in the Lidget Green area and premises in Shipley have also been raided.
Mr Dent said: "There appear to be links between the commercial cultivation sites we have come across in West Yorkshire and Vietnamese individuals.
"It is not just a Vietnamese problem. We have had white Europeans growing cannabis for a long time, but the latest phase appears to be Vietnamese.
"We don't know why there is this link now. The Metropolitan Police have had a Vietnamese problem for some time and maybe these people think they can go about their business in relative anonymity in our city centres."
Mr Dent said it was a relatively new link and the police wanted to stamp it out early.
Cannabis was downgraded from a Class B to a Class C drug in 2004, giving many the impression the drug had been decriminalised, said Mr Dent.
The reality is that anyone aged ten or older found in possession of cannabis can be arrested and face up to two years in prison.
Since February this year the drugs team in Bradford has seized cannabis with a street value totalling more than £1 million.
Mr Dent said: "We have been taking a very tough stance in Bradford for the last few months and this is to continue during the crackdown.
"What we saying to the public is that if they see anything suspicious to get in touch with Crimestoppers.
"What we do not want is criminal organisations moving to Bradford and West Yorkshire and thinking they can set up shop here and start growing the drug."
As part of the operation the public are being asked to get in touch if they suspect cannabis may be being grown in their area. Crimestoppers currently receives around 70 cannabis-related calls a week.
Mr Dent asked people especially to be wary of the following things:
- Curtains being drawn at all hours of the day.
- People coming and going at unusual times.
- People taking large numbers of plant pots into houses.
- Gardening tools left outside homes which do not have gardens.
Crimestoppers can be contacted on 0800 555 111.
e-mail: mark.casci@bradford.newsquest.co.uk
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