BEIJING - Chinese inspectors tracing new cases of contaminated milk have shut dairy firms in the northwest and seized 72 metric tons of milk powder tainted with melamine, an industrial compound that killed at least six children in 2008.

Nearly 100 metric tons of tainted milk powder may still be on shop shelves, the China Daily reported Monday in detailing the closure of a Ningxia firm that sold the product.

A number of cases of melamine in milk have surfaced in the past few months, some of which appear to have come from old batches of contaminated powder that was never destroyed despite a scandal that damaged the reputation of the Chinese dairy industry.

Tiantian Dairy Co Ltd in Ningxia was closed after it was found to have repackaged and sold 170 metric tons of melamine-tainted milk powder that it received as a debt payment, the China Daily said, citing the local government.

The paper did not say where the milk had come from. Last week, three people at a dairy firm in neighboring Shaanxi province were arrested for manufacturing or dealing in products laced with melamine, a compound commonly used in plastics or fertilizer but which can also be added to foods to show high protein levels in tests.

Another dairy firm, Ningxia Panda, was shut because of its ties to Shanghai Panda Dairy Co, which was closed late last year for selling products tainted with melamine.

There have been no reported deaths or illnesses from the latest batches of tainted milk. About 300,000 children sought medical treatment, many with kidney stones, in the 2008 scandal.

China executed two people in November for their role in the 2008 scandal that further sullied the made-in-China brand after a string of health and product-safety scares.

(Reporting by Yu Le and Lucy Hornby; Editing by Nick Macfie)