Chinese police have broken a crime ring that passed off more than $1 million in rat and small mammal meat as mutton, authorities said, in a food safety crackdown that coincides with a bird flu outbreak and other environmental pressures.

Authorities have arrested 904 suspects since the end of January for selling and producing fake or tainted meat products, the Ministry of Public Security said in a statement posted on its website on Thursday.

During the crackdown, police discovered one suspect surnamed Wei who had used additives to spice up and sell rat, fox and mink meat at markets in Shanghai and Jiangsu province.

Police arrested 63 suspects connected to the crime ring in a case valued at more than 10 million yuan ($1.6 million) in sales since 2009.

Despite persistent efforts by police, "food safety crimes are still prominent, and new situations are emerging with new characteristics", the ministry's statement said, citing "responsible officials".

Police confiscated more than 20,000 tonnes of fake or inferior meat products after breaking up illegal food plants during the nationwide operation, the ministry said.

Food safety and environmental pollution are chronic problems in China and public anxiety over cases of fake or toxic food often spreads quickly.

In April, many consumers lost their appetite for poultry as an outbreak of the H7N9 bird flu virus spread in China. Sales dropped by 80 percent in eastern China, where the bird flu has been most prevalent, although experts stress that cooked chicken is perfectly safe.

In March, more than 16,000 rotting pigs were found floating in one of Shanghai's main water sources, triggering a public outcry. Over-crowding at pig farms was likely behind the die-off and their disposal in the Huangpu river.

The public security ministry said police had confiscated more than 15 tonnes of tainted pork in Anhui province, although as much as 60 tonnes had been sold in Anhui and Fujian provinces since mid-2012.

But it was the rodent meat in particular that people couldn't stomach, with Internet users turning to the popular microblogging site Sina Weibo to vent their outrage.

"Rats? How disgusting. Everything we eat is poison," one user wrote. – Reuters