Rio Tinto has rejected BHP’s offer of a three-for-one share swap and a cash sweetener may now be needed.

According to London’s Sunday Times, financial advisers, Goldman Sachs and Citigroup, have already gone to China to sound out potential buyers.

While Chinese interests are unhappy with the idea of BHP and Rio merging, that country’s energy majors would be interested in BHP Billiton Petroleum’s Western Australian gas assets and its Gulf of Mexico oil projects.

The division is believed to be worth about $A30 billion, but some observers have suggested some international buyers might be prepared to pay more than $A45 billion for BHP Billiton Petroleum.

Speculation on the sale of this division resurfaces at regular intervals.

BHP's new chief executive Marius Kloppers has spoken of the importance of petroleum in the corporation’s "energy mix" and the substantial production and profit growth to come from BHP Billiton Petroleum as its big Gulf of Mexico projects come online.

But he has not yet said that the petroleum division is not for sale.