Chinese police have seized more than 3,200 tons of a new type of "gutter oil" made from decomposing animal fat and internal organs, said the Ministry of Public Security.

More than 100 people suspected of producing the oil were detained in a March 21 food-safety operation that covered two municipalities and four provinces, according to a statement issued by the ministry on Tuesday.

Police also closed down 13 underground workshops that were producing the oil, the statement said.

Previously, the phrase "gutter oil" referred to the reprocessing of used oil and even restaurant leftovers, which were then resold as cooking oil.

However, the new strain is made from meat that was past its sell-by date and of low quality, and even rotten animal fat and internal organs, according to the statement.

In October, residents in Jinhua city, East China's Zhejiang province, told local police that they often smelled a foul odor in an area on the outskirts and suspected that gutter oil was being made there, said the ministry.

After a five-month investigation, police found that the group, led by a man called Li Weijian, was producing illegal oil and had animal carcasses and organs in their workshop, said the statement.

The products were then sold to refiners in the provinces of Anhui and Jiangsu, and the municipality of Chongqing. The oil was then used by restaurants as a base for hotpots, the statement said.

Li's gang earned more than 10 million yuan ($1.59 million) between January and November 2011, according to the ministry.

Although China launched a nationwide campaign to fight practices such as this in August 2011, several food-safety scandals, including cases involving gutter oil and the illegal addition of pig feed to foodstuffs, still occurred.

"The campaign has made some achievements, but there is no well-organized mechanism for the collection, transportation and supervision of gutter oil in China," said a senior ministry official, who declined to be named.

A few criminals and even some businesses take advantage of those loopholes to make illegal profits by endangering the health of residents, according to the official.

The statement disclosed that the ministry has pledged to make greater efforts to safeguard food safety and combat any related offences.

Yi Shenghua, a Beijing-based lawyer who works for the Yingke Law Firm, said the circumstances of the transprovincial crime were extremely serious.

"The case involved a large sum of money and the products were sold across a wide area, including four provinces and two municipalities, which may pose a health risk to many people," he said.

"The new type of gutter oil uses rotten animal meat and fat as raw materials, which may result in extremely severe health problems," he said. "Judges need to consider the public reaction when sentencing those convicted of such crimes."

Those who engage in such activities should be punished severely, said Yi. "Unsafe food may affect thousands of people and even destroy the health of an entire generation," he added.

Hotpot devotee Fan Jing, a 27-year-old Chongqing resident, said she was not surprised by the case, and added that she will choose more expensive restaurants in future to minimize the risk.

"I can't live without hotpot, even though I know many restaurants use gutter oil," she said.