In documents filed with US regulators the mining giant told of $US800 million in fees associated with a $US55 billion loan facility, which was established to refinance debt from Rio's Alcan acquisitions and fund a proposed buyback of up to $US3.7 billion shares, according to The Sydney Morning Herald.

The documents also showed Chinalco's 12 per cent holding in Rio Tinto Plc may force BHP to sell its own assets rather than Rio's assets.

BHP said it would save around $US3.7 billion annually by merging its operations with Rio.

The merged group would produce about 710 million tonnes of iron ore annually from 2015, which is an 11 per cent increase compared to the two companies' separate plans that would generate 640 million tonnes each year, reported The Australian.

The paper said the documents showed BHP's bid was focused on Rio's iron ore operations in the Pilbara, which would be easier to expand that BHP's own operations in the region. BHP did not flag plans to boost production in any other commodity as a consequence of the bid.

The news comes as Rio prepares for industrial action at its Pilbara operations.

Exports from the miner's iron ore operations are expected to be hampered by strikes starting this Saturday, when train drivers will walk off the job, a union says.

This will be the first industrial action at the miner's Pilbara operations in Western Australia since 1992.

The company's 39 train drivers, members of the Construction, Forestry, Mining and Energy Union, voted unanimously in favour of the stoppage at a meeting in Perth last night, union mining division secretary Gary Wood said.

The drivers are seeking an annual pay rise of 4.75 per cent plus $20,000 a year for employees affected by Rio's plans to introduce driverless trains at its Pilbara operations by 2012.

BHP May Have To Sell Assets

BHP said "Compliance with any conditions and obligations imposed in connection with regulatory approvals could be affected, with any required divestments having to be made form BHP Billiton's portfolio rather than Rio Tinto's if consent of the Rio Tinto minority shareholders is required and cannot be obtained."

There has been talk that BHP would consider selling its Robe River Pilbara iron ore assets, in which Rio has a 53 per cent shareholders, according to The Australian Financial Review.

But aside from concerns this would ruin the synergies for the miners in the Pilbara, it now looks like it would be hard for BHP to promise such a sale without knowing how much of Rio it would acquire.