Workers at the world’s biggest copper mine in northern Chile on Wednesday filed a legal complaint against their employer for “anti-union practices” as their strike over pay went into its sixth day.

About 2,300 workers at the Escondida mine, in which Anglo-Australian mining giant BHP Billiton has a controlling stake, downed tools last week over unresolved contract disputes, including one about annual bonuses.

Workers want a bonus of $US11,000 linked to the company’s earnings. The company has offered $US6,000.

“Our lawyers have filed a complaint against La Escondida for anti-union practices in the workers’ court,” union leader Jose Vidal said.

A spokesman for the workers, Marcelo Tapia, told AFP that striking employees had been threatened with the loss of their jobs, and that union leaders had been prevented from entering the mine premises.

The strike could cost mine operators an estimated $US30 million ($A27.3 million) a day.

Chile is the world’s largest copper producer, with 5.6 million tons annually.

The Escondida strike comes after a strike earlier this month by workers at Chile’s state copper giant Codelco, when workers walked off the job to protest measures that they said were steps toward privatization.