BEIJING (AFP)--China said Monday it might undertake an antimonopoly review on a joint iron ore project between mining giant Rio Tinto Plc (RTP) and BHP Billiton Ltd. (BHP) after Rio rejected a Chinese bid for a massive investment.

Anglo-Australian Rio Tinto this month snubbed Aluminum Corp. of China Ltd. (ACH), or Chinalco, offer of a $19.5 billion cash injection in favor of a joint venture in Western Australia with bitter rival BHP Billiton.

"We think (the tie-up of the two companies) will definitely impact the global iron ore supply and therefore it is natural that Chinese companies are concerned," commerce ministry spokesman Yao Jian told reporters.

China is the world's largest iron ore importer while exports by the two companies account for 36% of the world's total supply, he said.

"Once the business operators' combined sales in China reached a certain level, the antimonopoly law will apply," he said.

China's antimonopoly legislation requires firms to get Chinese government approval before their merger if their aggregate global revenue exceeds CNY10 billion ($1.46 billion) or if revenue in China exceeds CNY2 billion.

Authorities will also review the deal if two or more of the firms each reported more than CNY400 million of revenue in China in the last fiscal year.

Sales into China generated $11.7 billion of revenue for BHP Billiton and $10.8 billion for Rio Tinto in the year ended 30 June 2008, according to the miners' Web site.

It was unclear how China might enforce its decision if it were to deem the joint venture in violation of its antimonopoly legislation.

Yao said the government hadn't yet had any application from Rio Tinto or BHP Billiton on the joint venture deal.

Separately, he said, the decision by Rio Tinto to reject the Chinalco offer was "a business decision," adding Chinese companies would "definitely continue" to seek to expand overseas and the pace w "even pick up in the future."

He said China would remain open to foreign investment and would "have no reason to take inappropriate measures" to retaliate.