China's Health Ministry has instructed food safety authorities in Hubei Province to investigate claims that milk powder has caused infant girls to grow breasts, a ministry spokesman said.

Ministry spokesman Deng Haihua said at a regular press conference that food safety authorities were already testing samples of milk powder made by Syrutra, a dairy company set up in Qingdao, a coastal city in east China's Shandong Province, in 1998.

Parents and doctors in Hubei were reported earlier this month voicing fears that milk powder produced by Syrutra had caused at least three infant girls to develop prematurely.

Deng said authorities were also conducting a medical investigation into the cause of the infants' conditions in consultation with medical experts.

He promised timely publication of the investigation results.

However, Deng said, premature breast development was usually a pre-puberty condition, with an incidence rate of two in every 1,000.

Causes for sexual prematurity of children were complicated and could be caused by a wide range of factors, and experts had no way to definitely determine if food or environmental factors were involved yet, he said.

Deng said a 2008 regulation banned sales and reproduction of products made from livestock under the influence of drugs, or those failing to pass health and quarantine inspection standards.

He said estrogen hormones were forbidden in milk powder products and the Ministry of Agriculture had formulated test procedures for estrogen hormones and had provided them to Hubei authorities.

The food safety supervision team in Hubei Province held a meeting Tuesday afternoon and decided to transfer the milk samples to central government authorities for testing. The samples are expected to arrive in Beijing Wednesday.

A parent of one of the victims in Wuhan, also with the surname of Deng, said she was concerned about her baby's health.

"I hope the test results will come soon," she said.