Criminal gangs are suspected of stealing pets for use in the barbaric illegal blood sport of dog-fighting, Scots animal campaigners say.

Warnings on Facebook and Twitter claim colour-coded stickers are being used to mark houses where there are dogs that are being stolen for use as bait animals in dog fighting.

Some owners have been posting concerns online while making attempts to trace their dogs.

One dog owner said his four-month-old female Lhasa Apso, Roxi, was stolen from Holmston Road, Ayr on March 24 by a man driving a black Ford Focus. "The worrying thing about this case of theft is that the man had bite marks all over his hands," said the owner. "Possibly from operating in dog fighting."

Last month, blobs of paint were found on gates and doors in South Ayrshire with owners concerned that a criminal gang was designating the location for the theft of dogs.

The house-marking warning was highlighted on Facebook by a number of concerned pet lovers who "shared" the notice.

It stated: "Keep your eye out for small coloured stickers on gates or doors. Gangs are marking how many dogs live there to steal and are using them for bait for dog fighting.

"Red stickers are for big dogs, yellow stickers are for medium breeds and pink for small breeds."

The notice says the markings were seen in Ayr and Maybole as well as numerous nearby villages.

Police say that while they are aware of reports on social media, they have had no reports from owners.

A Strathclyde Police spokeswoman said: "All we can do is alert owners to be vigilant and ask that people try not to leave dogs unattended in their garden where possible and report suspicious activity to police."

But Linda Ess, founder of Lost Dogs Scotland, whose organisation helps reunite owners with their lost or stolen pets, insisted there is a problem.

She said: "Every day, dogs are stolen here. Some are stolen and kept, some are sold on and some will be for dog-fighting. Anyone leaving their dog unattended is leaving their dog at risk."

She said some were giving their dogs away free through online trading sites in the mistaken belief they are going to a good home, but are, in fact, inadvertently, providing the pets as bait in dog-fighting.

"People are going to homes saying they are a family but are in fact involved in dog-fighting. You should never give away your dog free without checking out the home that it is going to go to and getting references."

Scottish SPCA, the animal welfare charity, is advising people to have their dogs microchipped. Its chief superintendent, Mike Flynn, said: "In our experience, dogs tend to be stolen either for breeding purposes or because they can be sold on.

"We always recommend that dogs are microchipped, which is a cheap means of legally proving ownership, often resulting in animals being returned to their owners."

Eileen and Michael Bray of Erskine say their German Shepherd cross Buster was brutalised by thugs who took him to use in dog-fighting. The dog was found last month having been mistreated and starved.

Mrs Bray, 38, said: "It seems they have been trying to encourage Buster into becoming an aggressive dog."