Santiago, Chile: Workers at the world’s largest copper mine, BHP Billiton’s Escondida in Chile, began an indefinite strike Thursday to demand pay increases, prompting concerns on world commodity markets.

About 1,200 employees kicked off the strike, with more to follow, said Carlos Allendes, spokesman for the miners’ union.

‘The legal strike is underway with the first shift not entering their workplace, and now it will be the second shift too,’ Allendes told AFP.

Escondida, a giant mine complex in the Atacama desert in northern Chile, produces about five percent of global copper output, some 927,000 metric tons (one million tons) a year.

Its 2,500 workers began shutting down machinery Wednesday in preparation for a strike that Allendes warns could be ‘long and hard.’

Ninety-nine percent of workers voted Tuesday to strike, after the collapse of negotiations mediated by the country’s labour directorate.

BHP Billiton, the world’s biggest mining company, has rejected workers’ demands for a seven percent raise and bonuses of 25 million pesos (around $39,000).

It is offering bonuses of eight million pesos, with no raise.

Like other miners, the Anglo-Australian company has been forced to cut costs as copper prices have slid in recent years, from a record high of $10,190 per metric ton in February 2011 to $4,318 per ton in January 2016.

The company has said it will suspend production for at least the first 15 days of the strike.

The workers, who have set up a protest camp outside the mine, say they have a war chest of $390,000 to sustain the action.

The union is vowing the longest strike since 2006, when workers walked out for 25 days.

The strike — and the threat of others at several top mines where contracts expire this year — have sent copper prices higher over fears of a shortage.

Chile is the world’s largest copper producer, supplying one-third of global output.

Copper is used primarily in power generation and for the transmission of electricity.