SYDNEY - MINING giant Rio Tinto on Friday confirmed a huge take-up for its record rights offering after investors snapped up the Australian portion, allowing it to pay off a large chunk of its heavy debt.

The dual-listed firm, whose share offer came at rock-bottom prices in Sydney and London, announced a 95 per cent subscription among Australian investors following a 97 per cent take-up in Britain.

Rio can now slash its debt bill incurred by last year's acquisition of Canada's Alcan. The offering, worth US$15.2 billion (S$22.1 billion), was the industry's biggest and the fifth-largest in history.

The Anglo-Australian giant announced the move last month along with an iron-ore joint venture with fierce rival BHP Billiton, snubbing a US$19.5 billion cash injection by China's Chinalco.

The new shares were issued at a price of A$28.29, a massive discount to the A$51.75 they closed at after Sydney trading the previous day.

Chinalco on Thursday said it had taken up its allocation to remain Rio's biggest shareholder, warning that it would keep a close eye on developments.

China has said it may oppose Rio's joint venture with BHP, aimed at merging their vast Western Australia operations, on anti-monopoly grounds.

Analysts also said the successful rights issue would strengthen Rio's hand as it looks to sell off assets. The miner dropped US$38 billion into the red with its purchase of aluminium company Alcan.

Australian investors bought 142,149,887 of the new shares, Rio said in a statement. The firm was in a trading halt while underwriters seek subscribers for the remaining 7,865,410. -- AFP