Investigators have found that a tainted milk incident in north-west China which killed three children appears to be a case of intentional poisoning and have detained a suspect, state media said.

Investigators found that nitrite, an industrial salt that can be deadly, was added to fresh milk from two dairies last week in Gansu province in order to harm people, the China Daily newspaper reported on Monday. A suspect in Pingliang, where the poisoning took place, had been taken into custody.

The three children who died were all under two, with the youngest being 36 days old, the China Daily said. An earlier report in the Pingliang News, a local government-run paper, said 36 other people, mostly children, became ill in the incident.

China's worst food safety scandal in recent years involved fresh milk, infant formula and other dairy products tainted with the industrial chemical melamine, which can cause kidney stones and kidney failure.

Melamine was widely used by Chinese dairy manufacturers to artificially boost protein levels and profits. It ended up killing six children and making more than 300,000 others ill in 2008.

Much of the food in China is still produced by backyard farms and small-scale manufacturers, which makes enforcing food safety standards difficult. A series of embarrassing problems, including the 2008 tainted milk scandal, prompted China to pass tougher food safety regulations and step up inspections, though many problems remain.

In a separate report on Monday, the China Daily said authorities in the central province of Henan had detained 95 people involved in a profit-seeking tainted pork scandal. The detainees allegedly made, sold or used pig feed laced with clenbuterol, a banned drug that causes pigs to convert fat to muscle quickly.

Clenbuterol is illegal because it can cause nausea, dizziness, headaches and heart palpitations in humans, but pig farmers like to use it because it helps yield more lean meat, which is costlier than fatty meat.

The investigation was launched after tainted pork was found being sold by Shuanghui Group, China's largest meat processor. Authorities traced the contamination back to an illegal chemical factory that sold raw clenbuterol to middlemen who mixed it with starch and resold it to farmers, who mixed it into their feed, it said.

Authorities found 18 tonnes of clenbuterol-tainted feed in Henan, and a random check of nine farms in the province found that 52 out of 1,512 pigs tested positive for the drug.