China will further step up its oversight of the markets to prevent the sale of unauthorized genetically modified seeds, an agricultural official said on Wednesday.

Liao Xiyuan, head of the Agriculture Ministry's Department of Science, Technology and Education, told a news conference that the ministry would focus on key agricultural seasons to prevent unauthorized seeds from flowing into the markets.

Liao's remarks follow a report from environmental nonprofit organization Greenpeace in January that claimed farmers were illegally growing genetically modified corn in northeastern China.

The organization claimed that 93 percent of samples taken from corn fields in five counties in Liaoning province last year tested positive for GM contamination.

China's agricultural authorities have to approve GM grains before they are marketed, with only domestic GM papaya and cotton having so far been approved for commercial cultivation.