There are fears contamination could be widespread throughout the food chain
The toxic chemical melamine is probably being routinely added to Chinese animal feed, state media has reported.
Correspondents say the unusually frank reports in several news outlets are an admission that contamination could be widespread throughout the food chain.
The melamine scandal began early in September, when at least four Chinese babies were killed by contaminated milk, and thousands more became ill.
The news led firms across Asia to recall products made from Chinese milk.
The problem widened last weekend when the authorities in Hong Kong reported that melamine had also been detected in Chinese eggs.
Four brands of eggs have since been found to be contaminated, and agriculture officials speculate that the cause was probably melamine-laced feed given to hens.
Melamine is high in nitrogen, and the chemical is added to food products to make them appear to have a higher protein content.
Several state newspapers carried reports on Thursday suggesting that the addition of melamine to animal feed was widespread.
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.
"We cannot say for sure if the same chemical has made its way into other types of food," the newspaper added.
10 Sept: 14 babies reported ill in Gansu province, cases reported around China
13 Sept: 19 people arrested
15 Sept: Beijing confirms first deaths from the contamination
22 Sept: Toll of ill babies rises to more than 50,000; head of China's quality watchdog resigns
23 Sept: Other countries start to test Chinese dairy products or remove them from shops
14 Oct: China orders the withdrawal of all milk produced more than a month ago
22 Oct: Hong Kong scientists find excessive levels of melamine in a brand of mainland eggs
31 Oct: Chinese media suggest melamine is routinely added to animal feed
The practice of mixing melamine into animal feed is an "open secret" in the industry, the Nanfang Daily reported.
Chinese officials have been criticised for initially covering up the melamine scandal - as they have in the past for other health scares.
Despite a nationwide campaign to raise food safety standards and reassure consumers, China's broken-down food safety inspectorate is still failing to catch and report lapses in standards when they happen.
Analysts say that Friday's news reports are an unusual departure for Chinese officials, marking what amounts to a tacit government admission that the problem could affect many parts of the food supply.