Authorities are continuing to investigate whether dozens of children in central China were used as test subjects in a US-China joint research project that included genetically modified (GM) rice.

The Chinese Center for Disease Control and Prevention (China CDC) said in an online statement that its researcher was not informed that the American side had used GM rice in the joint test, which was designed to study how children's bodies absorb and transform beta carotene.

Sources in the health department of Hunan Province, where the test allegedly took place in 2008, on Wednesday said "relevant officials" had gone to Beijing to participate in a joint investigation with the China CDC.

This came after local government officials in Hunan and the provincial CDC had publicly denied the claim. But a lead author of the research paper, published in the August edition of The American Journal of Clinical Nutrition, insisted that the study had been conducted with all regulatory approvals required by each country.

The paper claimed that Golden Rice, genetically engineered to be rich in beta carotene, is effective in providing vitamin A to children. The experiment involved feeding the rice to 24 children aged between six and eight years old in Hunan in 2008, according to Greenpeace, which broke the news in late August.

The paper's lead author is Tang Guangwen, director of the Carotenoids and Health Laboratory of Tufts University in the United States. While Tang insisted the testing had been approved, both the second and third author -- Hu Yuming with the Hunan CDC and Yin Shi'an with the China CDC -- denied involvement in the GM rice research project.

The two toed the line of Hunan authorities, which on Sunday said the China CDC had conducted tests on children in the province in 2008, but these tests were meant to study children's bodies transformation of beta carotene in vegetables to vitamin A, and did not include GM food.

"I am totally clueless," Hu told reporters.

"I am aware of the vegetable and beta carotene transformation parts of the paper but know nothing about the part involving Golden Rice," Yin added.

The China CDC statement said the American project was signed by Tufts University and Zhejiang Academy of Medical Sciences and was led by Tang.

Yin, who led a project approved by the China CDC on the transformation of beta carotene, combined his on-site tests with that of Tang, as both covered studies on beta carotene in spinach, the statement said.

But the statement cited Yin as saying he did not know whether Golden Rice had been involved in the American study.

GM food is controversial, as there is still no consensus on whether or not it is harmful to the human body.

According to the Greenpeace website, it is simply not known whether genetically engineered crops are safe for human or animal consumption. Independent scientific studies on the matter are severely lacking, it said.

"Children and infants are particularly sensitive to any possible health effects of genetically engineered rice," it added.

"Absolutely disgusting!" said a comment posted under the Greenpeace article. "As a mother, I am outraged!"

The Chinese government introduced a regulation as early as 2001 to ensure the safety of GM food, with strict provisions for researching, testing, producing and marketing such products.

According to the regulation, parties conducting GM agricultural experiments should have certain qualifications and form a panel to be in charge of the safety of the experiments.

It also provides that any China-foreign GM agricultural experiment should be approved by the government's agricultural departments.

An official with the Hunan provincial government's agricultural bureau in charge of GM-related issues said the office did not receive any application for a GM food experiment and did not give any approval.

"Only after we know about the truth of this experiment can we talk food safety," Shanghai-based Oriental Morning Post wrote in a commentary. "The Ministry of Health and the Ministry of Agriculture need to disclose the truth."