Melbourne: BHP Billiton Ltd's coal miners in Australia went back on strike after failing to agree on pay and work conditions with management, further disrupting operations at the largest coal exporter.

"Strikes are continuing and now we are considering an escalation," Stephen Smyth, a division president at the Construction, Forestry, Mining and Energy Union in Queensland, said.

Stoppages that disrupted operations at the three BHP Billiton Mitsubishi Alliance mines on September 10 will continue until at least today, Smyth said. The action may be extended to BMA's four other mines, pending a member vote later this week, he said.

About 3,500 coal miners have taken part in rolling work stoppages that began in June, the first in a decade at BHP's Australian coal mines, after they rejected BHP's initial offer for a new contract.

The Melbourne-based company said it will allow all workers at the mines to vote on its proposals at a meeting at the end of September.

"We have reached an impasse in negotiations" after a meeting with unionists on September 10 failed to reach an agreement, BHP spokeswoman Samantha Stevens said. "We are keen for employees to take the time in the coming weeks to review and understand the agreement in detail, before the end-of-September ballot."

BHP fell 3.9 per cent to A$36.45 at the close on the Australian stock exchange, reaching its lowest closing price since May last year. The benchmark stock index fell 3.7 per cent.

Prices, profits

Labour unions globally are stepping up demands for higher wages and improved conditions as surging commodity prices swell profits at mining companies including BHP. About 1,200 workers at Freeport-McMoRan Copper & Gold's Peruvian copper mine last week began the first of a series of strikes planned over pay increases. More stoppages are planned if no agreement with management is reached this week, union official Leoncio Amudia said September 9.

BMA is equally owned by BHP and Mitsubishi Development. It directly employs more than 4,800 people, according to its website. Almost all the coal mined at the BMA sites, with annual capacity of 58 million metric tonnes, is shipped overseas for steel production, it said.

BMA's initial offer for the workers for a 5 per cent pay rise in addition to a A$5,000 (Dh19,000) bonus was rejected in June, when they began the industrial action.

A full-day strike may cut output by 130,000 metric tonnes a day from the mines, Melinda Moore, an analyst at Credit Suisse Group AG, said in a June 27 report.