Brazil's Vale and London-based Rio Tinto, the world's largest and second-largest iron ore mining companies have agreed to become members of a Chinese iron ore trading platform China Beijing International Mining Exchange (CBMX).

Both miners became members of CBMX after Australia's third-largest iron ore producer Fortescue Metals Group joined the Beijing-backed exchange last week.

"We welcome the development of CBMX as it gives us a new option for selling any available tons to China, over and above those already contracted," Alan Smith, president of Rio Tinto Iron Ore Asia, said in a statement.

"We look forward to the exchange developing into a transparent, independent, efficient and sustainable iron ore trading platform supported by broad market participation," he added.

Rio Tinto's larger rival BHP Billiton is yet to sign in, as is London-based diversified miner Anglo American.

A London-based spokesman for BHP Billiton last week said that the company was "studying participation and are in principle supportive of any platforms that support market transparency and liquidity."