Hong Kong police said Sunday they had launched an animal cruelty investigation after a man joked online about killing a dog by trapping it inside a churning washing machine.

An image uploaded to Facebook by a man calling himself Jacky Lo shows a small, white dog attempting to climb out of the machine, while another apparently portrays the animal struggling to keep its head above water with the washing cycle in full flow.

A police spokeswoman told AFP that a case of suspected animal cruelty was under investigation.

"A picture has been posted onto the internet about an animal suspected of being tortured. It is a case of animal cruelty," the spokeswoman said.

"Lo" said he posted the pictures to show "a super quick way to bathe a doggy".

In reply to another user who asked if he had killed the dog, he replied in comments under the pictures: "Yes (it's dead). Do you wanna see?"

An online petition which had urged the authorities to take action after the pictures were posted last month and went viral last week has drawn nearly 20,000 signatures.

"If you really are a pet lover you will not (be) so cruel to kill such a cute little dog. Come on you are a man, not a beast," wrote Belinda Heng, a signatory of the petition.

"Stop! Dogs also have life!!" a user named Vivien Tsui posted under the online appeal.

Despite living in a densely-populated city filled with tiny apartments, many Hong Kong residents are passionately devoted to their pet dogs.

Last month, protests were held at one of the city's subway stations after rail workers failed to stop a stray dog from being run over by a train.

Under Hong Kong laws, animal cruelty offences carry a maximum sentence of three years in prison and a fine of HK$200,000 ($25,800).

Police said no arrests had been made so far.

Rebecca Ngan, a spokeswoman for local animal protection group the Society for the Prevention of Cruelty to Animals, told AFP: "It's truly horrific. No one should do that. Police should arrest him."