Aluminium Corporation of China (Chinalco), the state-owned aluminium group, was keeping its options wide open after BHP Billiton formally launched a $147bn bid for mining rival Rio Tinto, which was rejected late Wednesday afternoon.

Chinalco, which last week stunned industry observers by snapping up a 9 per cent stake in Rio Tinto in an audacious dawn raid, said in a statement on Wednesday that it was watching developments in the bid battle.