Thirty-one samples of Chinese milk powder provided by 20 companies were found tainted with melamine after new testing was conducted, according to data seen on Wednesday on China's food safety administration's website.

Out of the 20 companies identified on the administration's list Wednesday as producing goods with melamine, 15 had not been named after previous testing. The contaminated samples were mostly milk powder products for adults.

Melamine contamination has been blamed for the deaths of four children and kidney ailments among 54,000 others. More than 13,000 children have been hospitalized and 27 people arrested in connection with the contamination.

It was a national holiday in China and product safety officials could not be reached for comment.

Melamine, which is high in nitrogen, is used to make plastics and fertilisers and experts say some amount of the chemical may be transferred from the environment during food processing.

But in China's case, suppliers trying to boost output are believed to have diluted their milk, adding melamine because its nitrogen content can fool tests aimed at verifying protein content.

Levels of melamine discovered in batches tested varied widely, from as much as 6,196 milligrammes per kilogramme to as little 1.3 milligrammes per kilogram. Chinese health officials have said no harm comes from consuming less than 0.63 milligrammes per kilogramme.

In the most recent tests, nine of the batches containing melamine were produced by the company at the center of the scandal, Sanlu, a 43 per cent stake of which is owned by New Zealand dairy cooperative Fonterra. No date for the testing was given.

The scandal was worsened by an apparent cover-up by companies involved and the ignoring by safety officials of tips and warnings from parents and doctors. Top Sanlu executives and government officials in the northern city of Shijiazhuang, where the company is based, have been forced to resign. -- AP