Global miner Rio Tinto warned on Monday that further cracks could be emerging in global commodities markets as the economies of Europe and the U.S. waver, with its customers increasingly cautious on the outlook.

Still, the world's second largest miner of iron ore and a large producer of copper, coal, aluminum and other industrial staples, said it was able to sell all the commodities it could produce.

The company's comments matched rival BHP Billiton , which earlier this month turned slightly more bearish on commodities demand, warning that some buyers were facing tighter access to credit.

In a statement ahead of an investor briefing, Rio Chief Executive Tom Albanese said continuing stresses in the euro zone and a weaker outlook for the U.S. economy were affecting customer sentiment, which had become more negative in recent months.

For the near term I am concerned about the general softening of prices when we continue to see cost escalation and strong currencies in Australia and Canada, Albanese said.

But while there are signs of nervousness, we believe the impact of current economic concerns on our business is manageable, unless financial markets substantially deteriorate, he said.

At the same time, Rio said it had approved $14 billion for projects in 2012 and said that could increase.

It also said it was raising its iron ore expansion target by 20 million tonnes to 353 million tonnes a year by the first half of 2015.

Albanese said prices for copper, coal and other Rio products were holding up with the exception of aluminum which is now priced well below the industry's marginal cost of production.

London Metal Exchange-traded three-month aluminum ended at $1,993 a tonne in the last session, close to its lowest since July last year of $1,982.25.

In response to toughening market conditions in aluminum, Rio is already making plans to permanently close its Lynemouth smelter in the UK.

It has also put its Australian and New Zealand smelters up for sale and placed its Sebree plant in the U.S. under review.

(Editing by Lincoln Feast)