Global miner Rio Tinto reported a 35 percent jump in first-half profit on Thursday, but it missed market expectations and sweetened the result with a $2 billion expansion of its existing share buyback program.

Booming iron ore sales to China helped propel underlying profit to $7.8 billion for the six months ended June, a record for the first half but short of analysts' consensus forecast of $8.03 billion and also below the previous half-year's result.

The numbers look a bit weak... It's not very encouraging, said resources analyst Hayden Bairstow of CLSA.

Rio Tinto also boosted its interim dividend to 54 U.S. cents per share from 45 cents a year earlier.

Rio Tinto joins major rivals Anglo-American Plc and Xstrata Plc in overcoming spiraling costs and restive labor unions to report profit growth of a third or more over the past week, thanks to surging commodity prices.

But with the global economy on edge, bedeviled by debt crises in Europe and the United States, the outlook remains uncertain, with investors worried about a return to global financial turmoil and a knock-on effect for commodity prices.

We remain positive for the remainder of 2011 and into 2012, in particular given the context of the industry struggling to bring new production onstream, Chief Executive Tom Albanese said in the results statement.

However there are important risks to this outlook related to the pace of credit tightening in developing countries and the threat of financial crises arising from sovereign debt problems in Europe and the United States which could destabilize commodity markets.

Iron ore net earnings, which accounted for 78 percent of Rio Tinto's group profit, surged 44 percent in the first half, while aluminum profits edged 3.5 percent higher.

Earnings from the energy division, which includes coal and uranium, tumbled 39 percent, partly reflecting flooding at its Australian collieries and at the operations of its Australian uranium subsidiary, Energy Resources of Australia .

The strong Australian dollar also weighed heavily on Rio Tinto's earnings.

(Reporting by Mark Bendeich; Editing by Balazs Koranyi and Ed Davies)