The Anglo-Australian miner is affected by movements in the US currency because most of its products are sold in dollars while the cost of mining its ores is denominated in Australian or Canadian dollars, or South African Rand.

In the first half of this year, those three currencies rose by 10% against the US dollar.

"The effect is that our margins are affected by currencies," chief executive Leigh Clifford told the BBC's World Business Report.

Hedging

Mr Clifford acknowledged that in many situations it would be possible to hedge, by using derivatives or other financial instruments to reduce a company's currency exposure, but this was not always possible.

"The assumption is that you know where [currencies are] going and that's pretty easy in retrospect, but not so easy in prospect," he said.

Besides, "we found that, in effect, we have a natural hedge", he said.

"Typically with a strengthening of the US dollar or the Canadian dollar, that is associated with higher commodity prices.

"We've seen a little bit of that in copper and aluminium," he said.

Rio Tinto has also enjoyed strong iron ore sales in recent months, essentially due to strong demand from China.

"There is no other industry that I am aware of where China is having such a huge dominant effect, as we are seeing in the mining and metals sector," chairman Robert Wilson said.

But performance of the company's energy division was "exceptionally poor", said Numis analyst John Meyer.