A new craze in which animals are sold in "mystery boxes" has caused outrage in China after a number of puppies were found dead inside.

The "blind boxes" have become a popular trend in the country, with people buying packages without knowing which animal they will receive through the post.

But on Monday, animal welfare activists made a distressing discovery when they found 160 small dogs and cats inside a truck in Chengdu in southwestern China's Sichuan province.

They found that the packages - which were disguised as regular express deliveries - contained a distressed puppy or kitten.

Chengdu Aizhijia Animal Rescue Centre said the vehicle they discovered was carrying 160 animals, all under three months old, in terrible conditions - and a number of them had died.

The latest mystery animal box craze has caused outrage in China
The latest mystery animal box craze has caused outrage in China

Disturbing footage posted on social media by the animal rescue group shows boxes piled up to the ceiling of the truck.

They wrote: "The cargo box is full of screams from cats and puppies."

The rescue organisation added: "We could hear them crying in discomfort.

"If the back door of the vehicle had been completely shut and no air was circulating, they would have suffocated to death."

Volunteers tried to help the animals during the night, feeding them glucose water with syringes through holes in the boxes, a member of the group surnamed Zhou told Sixth Tone.

Sadly, they could not save all of the animals and by Tuesday four of them had died.

Volunteers found 160 cats and dogs packed in tiny boxes in a truck
Volunteers found 160 cats and dogs packed in tiny boxes in a truck

The organisation said: "The dead bodies of several pets were tossed on the streets by ZTO Express [delivery company] workers."

Today, the rescue centre said they managed to bring the animals to their base and 38 were still receiving medical treatment.

The courier company involved in the shipment of the animals, ZTO Express, apologised and said the person in charge of delivery safety in the region has been suspended.

The company added it has launched further training regarding postal safety and national animal protection.

In China it is illegal to transport live animals, but "blind boxes" have become increasingly popular, according to state media.

There have been reports of similar boxes containing animals such as tortoises, lizards, hamsters and rats being sold on shopping websites Taobao and Pinduoduo.

In most cases, customers cannot choose a specific breed nor return the animals.

Puppies and kittens were in poor conditions and needed medical treatment after being rescued by an animal organisation
Puppies and kittens were in poor conditions and needed medical treatment after being rescued by an animal organisation

The animals found this week were ready to be dispatched to locations throughout China, travelling up to 1,000 miles, according to reports.

The illegal practice caused outrage in China, with numerous people criticising it on social media app Weibo.

A person posted: "Reject live animal transportation! The stench of corpses produces all sorts of germs that contaminate surrounding parcels... This is clearly an illegal act that is being done for personal monetary gain."

Another Weibo user wrote: "Buyers, sellers, the retail platform, and the couriers are all guilty. I can't imagine how anyone can do this."

China has come under harsh scrutiny because of its treatment of animals, with campaigners calling for an end to animal abuse.

In March, we published an investigation about fur farms, packed full of sick, distressed and horrifically injured animals.

Grim scenes were filmed at 13 farms in China, the largest fur producing country in the world.

At one location in China, raccoon dogs were found being so ineptly electrocuted that experts say they will have been rendered paralysed but still conscious, experiencing a slow, agonising death from cardiac arrest.

Rows of foxes were also captured repetitively spinning and pacing in their tiny, barren, wire cages, the classic symptoms of mental decline from environmental deprivation.