Chinese steel mills should buy more iron ore using spot trading platforms to reduce their exposure to index pricing, which Beijing feels is being manipulated, an industry official said on Wednesday.

Wang Xiaoqi, vice-chairman of the China Iron and Steel Association, said big miners were reducing volumes sold by long-term tenders in order to push up global index prices.

"The miners are only offering very small amounts by tender in order to influence prices and this is not fair towards long-term contracts," he said at an industry conference in Qingdao.

Iron ore is priced via spot benchmarks published by Platts and Metal Bulletin. The data providers calculate daily prices based on collected spot transactions for imports, mainly in China, which buys around 60 percent of the world's iron ore.

Those benchmarks are then used in pricing long-term contracts through which big steel mills, such as Baoshan Iron and Steel (600019.SS), secure their supplies from the miners.

It is not the first time that China has attacked the process by which its biggest commodity import by volume is being priced.

In March, the country's top economic planning agency accused "the three major miners and some traders" of manipulating iron ore prices.

The National Development and Reform Commission (NDRC) did not name the miners, but the biggest are Vale (VALE5.SA), Rio Tinto (RIO.AX) (RIO.L) and BHP Billiton (BHP.AX) (BLT.L), between them producing roughly two-thirds of the world's output.

BHP denied the allegations while Rio Tinto and Vale declined to comment.

At the conference, Wang urged iron ore suppliers to release more ore to sell on the platforms, saying it could set the benchmark price for the whole industry if the daily transaction volume reached 7,000-8,000 tonnes per day.

China has been trying to encourage more trade on its platform run by China Beijing International Mining Exchange (CBMX), which was set up in May 2012, but trading on the platform has remained relatively low.

Wang said the use of index prices to set contracts was "irrational", adding that some indexes only use spot tenders for that particular day to decide prices, making them "suspicious".