* Rio Tinto hikes annual dividend 12 percent
* Rio slashes net debt to $12.5 bln, well below forecasts
* Rio Tinto H2 underlying earnings fall 30 pct
* Rio Tinto UK shares up 2.6 percent (Adds analyst quote, updates shares)
By Sonali Paul
MELBOURNE, Feb 12 (Reuters) - Global miner Rio Tinto handed $2 billion in capital to shareholders on top of a higher than expected dividend on Thursday, despite reporting its worst half-year profit in two years.
The bumper return came after the world No.2 miner cut costs, capital spending and debt to shore up its cash flows against collapsing commodity prices, with the steepest slide in its biggest business, iron ore.
The biggest surprise was Rio managed to cut net debt to $12.5 billion, sharply below forecasts, and flagged it would cut capital spending in 2015 to less than $7 billion, instead of $8 billion, both factors that will give it flexibility to continue returning cash to shareholders.
Rio Tinto Chief Executive Sam Walsh, under pressure to please investors to ward off a renewed takeover approach from rival Glencore Plc, said he was confident the company would continue to generate sustainable returns for shareholders.
"2015 will be a tough year for the industry, but we are ready to respond to any such conditions," Walsh told reporters on a conference call from London.
"The strong balance sheet is a key competitive advantage, providing a wide range of options in the future regardless of market conditions," he said.
Underlying earnings for the six months to Dec. 31 fell 30 percent to $4.19 billion from a year earlier, based on Reuters calculations off the full-year result, but were well above analysts' forecasts for $3.76 billion.
Annual iron ore earnings fell 18 percent, but were stronger than expected. The world's lowest cost producer ramped up output, flooding the market and driving down its average prices for the year by 30 percent, which forced higher cost producers to cut volumes.
Rio increased its full-year dividend by 12 percent to $2.15, which was higher than the $2.12 the market was expecting.
It said it would return $2 billion through a buyback of its Australian and UK-listed shares, which was in line with what analysts had expected.
Profit from Rio's long-suffering aluminium division more than doubled in 2014 to $1.2 billion, overtaking copper as the group's second-biggest earner.
"It's a pretty strong set of results, and they're giving the market what they're wanting," said Ric Ronge, a portfolio manager at Pengana Capital.
Rio's UK-listed shares jumped 3.4 percent by 1431 GMT, outperforming a 2.6 percent gain in the mining index .
"Interesting brinkmanship from Rio this morning, showing its broadside to Glencore," said Nomura analyst Matthew Kates in a note. "Let's see how Glencore respond."
Rio Tinto rejected a tentative approach from smaller rival Glencore in July and Glencore said in October it was no longer pursuing a dialogue with Rio Tinto. Some analysts expect another takeover attempt by Glencore, which swallowed miner Xstrata.
Anglo Australian Rio is the only one of the world's big five miners in a position to return capital at this point, after Glencore made a $1 billion return last August.
Walsh and Chief Financial Officer Chris Lynch did not make any promises about further buybacks a year from now, but said the company should have enough flexibility to consider one.
Arch rival BHP Billiton has taken similar steps to shore up cash flows against sliding iron ore, copper and coal prices, but the world's biggest miner, which also has an oil and gas business, has had to put off any plan to return capital following a halving of oil prices since June.
Rio Tinto's Australian shares have fallen 10.3 percent over the past year against a 9.8 percent rise in the broader market, but have outperformed BHP.
Anglo American Plc, the smallest of the top five global miners, is due to report its annual results on Friday. (Additional reporting by James Regan in SYDNEY and Eric Onstad in LONDON; Editing by Richard Pullin and Susan Thomas)