Rio Tinto and BHP Billiton are amassing vast copper holdings in a push to capture a greater chunk of the $140 billion world market, apparently aiming to squeeze out high-cost producers just as they did in the global iron ore business.

Separately and in joint ventures, Rio and BHP intend to mine millions of additional tonnes of copper, despite seeing an oversupplied market for the next few years.

"For both companies, this is about wielding the greatest influence possible over the global marketplace," said Gavin Wendt, senior resources analyst for Sydney-based consultants MineLife.

"Having said that, unlike in the highly concentrated iron ore space where the focus is squarely on one market owned in large part by Rio and BHP - China, copper is sold much more widely, leaving room for smaller producers to stay in the game," Wendt said.

Several smaller producers contacted by Reuters declined to comment, saying it was too early to gauge the impact of the expansions.

There have been no suggestions that BHP and Rio are working in concert to seize overriding control of global copper supply.

A worldwide supply surplus of 300,000 tonnes is forecast in 2015 by Australia's Bureau of Resource and Energy Economics, equivalent to half a year's output by South Korea.

Rio's copper division head, Jean-Sebastien Jacques, told investors in December a shift back to a deficit in the market was unlikely before 2018 and that the company was now striving to "gain a clear advantage" against competitors and "build a business model that creates value through the economic cycles".

BHP sees copper usage growing sharply in the next decade, partly on the back of a shift to renewable energy sources requiring copper-intensive apparatus. The metal has been designated a core pillar of growth by BHP's board.

"The market has been generally cautious for 2015-16, but beyond that, most are comfortable with the long-term story," said UBS commodities analyst Daniel Morgan.

While it is unlikely Rio and BHP will hold the same degree of dominance over copper that they do in iron ore - for the foreseeable future Codelco, Glencore and Freeport McMoran will remain bigger producers - their influence on global supply will be enhanced.

The strategy of mass output introduced this year aimed to flood the world with cheaply mined iron ore and drive prices down.

The result in iron ore so far, where Rio and BHP now control half the sea-traded market, has been the elimination of some 125 million tonnes of annual capacity - a tenth of world demand - by higher-cost producers.

The drive in copper could give BHP and Rio an advantage over rival Vale of Brazil, whose exposure to the red metal is less than half that of BHP and Rio and stagnating as it concentrates on its main iron ore business.

In regions as diverse as North and South America, Australia and Mongolia, BHP and Rio are digging new mines or expanding old ones, some requiring billions of dollars of investment, others hardly anything.-Reuters