BEIJING (AFP) — China's state-run press warned Friday a toxic chemical found in eggs and milk was likely being mixed into livestock feed, in the clearest official indication yet that other foods may be contaminated.

"The feed industry seems to have acquiesced to agree on using the chemical to reduce production costs while maintaining the protein count for quality inspections," the state-run China Daily said in an editorial.

Authorities in the eastern metropolis of Shanghai have already ordered that more than 100 fish farming enterprises in the city be tested to see if their feed is tainted with melamine, the Shanghai Daily reported.

It was one of the first reports warning seafood may also be laced with the chemical that made headlines in August after authorities admitted it had been mixed into milk.

The contaminated milk resulted in the deaths of four babies from kidney failure and the sickening of 53,000 others.

Hong Kong authorities reported last weekend that melamine had also been detected in Chinese eggs, leading to concerns the chemical was much more prevalent in China's food chain than initially believed.

Melamine is an industrial chemical normally used to make plastics and can lead to severe kidney problems if ingested in large amounts by humans.

But, after an initial cover-up by Chinese authorities, it was eventually discovered that the chemical was being routinely mixed into watered-down milk to give it the appearance of being protein rich.

Following the egg revelations, authorities are now investigating whether mixing melamine into livestock feed for the same reason is also a widespread practice.

"We cannot say for sure if the same chemical has made its way into other types of food," the China Daily editorial said.

"We hope it has not. But if fodder can be confirmed as the source of contamination for both the eggs and milk, action must be taken to check how widespread the use of this chemical is in the fodder industry."

Meanwhile, the Chinese company blamed for selling the original batch of tainted eggs to Hong Kong is suing its feed provider, according to the official People's Daily newspaper.

The brief report said an investigation by the company, Hanwei, found that livestock feed provided by Xinmin Mingxing company contained melamine.

Both companies are based in the northeastern province of Liaoning.

Meanwhile, state media also reported on Friday that authorities in southwestern China failed for more than a month to raise an alarm over an epidemic of maggots in mandarin oranges.