Analysts said the acquisition was a possible defensive move to prevent, or at the least complicate, any tie-up between Rio Tinto and BHP Billiton, two of the biggest miners and suppliers of raw materials, which China needs to drive its booming economy.

Beijing has voiced concern about a BHP-Rio Tinto combination as the merged company would control a major part of the world's iron ore supplies.

China's largest steel company, Baosteel, has even called on the Australian government to prevent such a deal.

The joint acquisition announced Friday was worth 14.05 billion dollars (9.4 billion euros), with the Chinese contribution "the largest overseas investment ever made by a Chinese enterprise," the official Xinhua news agency reported.

BHP Billiton offered a merger with Rio Tinto last year, valuing its Australian peer at more than 130 billion dollars.

Rio Tinto rejected the proposal but there has been persistent speculation BHP would come back and sweeten its offer.

"A BHP-Rio merger puts (China's) industries in a very disadvantaged position," said analyst Philip Chan, research head at CAF Securities, a unit of the Agricultural Bank of China.

"If there's anyone who would like to see the merger to fail that would be China."

Chinalco and Alcoa said they did not intend to launch a full takeover bid but analysts noted that the companies reserved the right to do so.

"Chinalco may or may not bid for Rio, but it already achieved one thing, which is to make it difficult for BHP to pursue Rio," said Michelle Leung, an analyst at Tai Fook Securities in Hong Kong.

A London-based spokeswoman for Chinalco rejected suggestions the move was designed to block BHP Billiton's ambitions.

"The reason for buying (the stake) is not for that reason," she said.

"Chinalco is going into this because they are very confident of the long-term prospects of the mining cycle. They have seen the mining bubble happen and they would like this transaction to increase and diversify their exposure to the sector."

Opposition to a BHP Billiton-Rio Tinto tie-up was widespread from other clients also worried that it would have too much control over key commodities which have soared in price over the past several years.

Rio Tinto said the price paid for the 12 percent stake in the company confirmed its initial reaction to BHP Billiton's offer that it was too low.

Chinalco and Alcoa paid 60 pounds sterling (30 dollars) a share for Rio Tinto, compared with the 50 pounds valuation in BHP Billiton's proposal.

In midday trade in London, Rio Tinto was quoted at 56.67 pounds.

China has flexed its increasing muscle in the global steel industry repeatedly in recent years as it battled Rio Tinto, BHP and Brazil's Companhia Vale do Rio Doce -- the big three suppliers -- to keep iron ore prices down.

In 2005, China's steelmakers and government were furious after iron ore prices were hiked 71.5 percent and they accused the big three of acting like a monopoly.

Founded in 1873 and headquartered in Britain, Rio Tinto became a leading aluminum producer last year by acquiring Canada-based Alcan for 38.1 billion dollars.

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