An upmarket Manhattan pet store has come under fire after an undercover investigation exposed abuse by members of staff.

Sick puppies at the Chelsea Kennel Club - including those with pneumonia, breathing problems and bloody diarrhea - were either denied prompt treatment to save costs or given the wrong medications, the Humane Society said.

Unsuspecting buyers were then duped into buying the animals thinking they were healthy, when in fact the store was covering up past medical and behavioral issues, the two-month probe found.

Members of staff were also caught on camera hitting the dogs with towels and handing them roughly, while others pinned animals down and put hands over their muzzles as part of 'dominance training'.

An undercover investigation by the Humane Society uncovered shocking abuse by staff at Chelsea Kennel Club, an upmarket pet store in Manhattan, including a sick dog being shaken so it will swallow its medicine

In another clip a member of staff is filmed hitting a puppy over the head with a towel in order to stop it barking. The store owner, Dana Derragh, said the woman has now been fired

John Goodwin, senior director for the Stop Puppy Mills campaign, said: 'From puppies with open surgical wounds on their bellies to a dog who could barely breathe because she was suffering from pneumonia, our investigator witnessed shocking disregard for the care these puppies need.

'The retail pet industry has a system-wide problem that begins with cruel puppy mills, continues with the way they ship baby animals across the country in cramped quarters, and ends with consumers often being sold sick animals at an inflated price.'

Video footage shows a Pomeranian with conjunctivitis that made its eyes swell shut, an underweight French bulldog, an English bulldog with pneumonia and a shiba inu with bronchitis.

Pet store staff the removed paperwork detailing abnormal veterinary findings from several puppies' folders before the animals were sold to unsuspecting buyers.

Many sick animals were also filmed at the store. But rather than treat them with the appropriate medication, one member of staff orders the investigator to use bird medicine

One Pomeranian was suffering from a bad conjunctivitis infection that had caused its eyes to swell shut. The Humane Society said some of the dogs were denied prompt care in order to keep down costs

Some of the sick dogs fetched more than $4,000 when they got better and were sold as healthy, the report says.

When confronted about this, sales staff said the store owner had instructed them to do it, despite it being against New York law.

Secret shoppers were also sent to the store to purchase two puppies, one of which had been identified as troubled, and another that was sick.

The first puppy had reportedly bitten her former buyer who then returned her to the store, but the salesperson did not reveal this to the shopper at the time of purchase.

Regarding the other puppy, two different sales people told the secret shopper she had 'a normal' or 'typical' cough, and one said the puppy had 'got it a few days ago.'

In fact, the animal had actually been sick in the store for weeks, according to the undercover investigator.

Even when the dogs were treated, their previous infections and abnormal veterinary results were hidden from their eventual buyers, which is against New York Law

Another French Bulldog puppy was filmed looking emaciated with its ribs clearly visible

She was taken to a vet within 24 hours of being purchased, who diagnosed her with bronchitis and later pneumonia. She may now have permanent lung scarring.

During the investigation, The Humane Society contacted the New York State Attorney General's and the New York City Mayor's offices about some potentially illegal practices. Both have vowed to look into the pet the store.

Protesters burst into the store on Tuesday, chanting anti-abuse slogans and getting into an argument with two men browsing the store, the New York Daily News said.

Dana Derragh, the store owner, denied any wrongdoing when she was confronted by the newspaper, saying she 'loves animals' and is a vegetarian.

Derragh added that she has fired two members of staff over the footage, including the one seen hitting the dog with a towel, saying she had no idea what had been happening at the store.