A new trend in China in which animals are sold in “mystery boxes” has caused outrage after multiple puppies were found dead inside.

The so-called “blind boxes” have become a popular trend in which people buy packages without knowing which animal they will receive.

This week, animal welfare activists made a horrifying discovery when they found 160 small dogs and cats inside a truck in Chengdu in southwestern China’s Sichuan province.

The packages were disguised as regular express deliveries, but each one 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 and in terrible conditions. A number of them had already died.

The animal rescue group shared disturbing footage on social media showing the boxes piled up to the ceiling of the truck.

“The cargo box is full of screams from cats and puppies,” they wrote. “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,” they added.

Volunteers tried to help the animals throughout the night. They fed the animals glucose water with syringes through holes in the boxes.

Unfortunately, they could not save all of the animals. By Tuesday, four of them had died.

“The dead bodies of several pets were tossed on the streets by ZTO Express [delivery company] workers,” the organization said.

The rescue center was able to bring the rest of the animals to their base. 38 are still receiving medical treatment.

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

The company also said that it has launched additional 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 are not able to choose a specific breed. They are also not allowed to return the animals.

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 criticizing it on social media app Weibo.

“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,” one person posted.

“Buyers, sellers, the retail platform, and the couriers are all guilty. I can’t imagine how anyone can do this,” another said.