SANTIAGO - Workers at Chile's Spence copper mine on Thursday entered the third day of their strike, which has hurt output at the BHP Billiton unit.

A BHP spokesman in London told Reuters the mine's output of copper cathodes was lower after mining and agglomeration activities stopped. He declined to say by how much.

In an e-mail answer, he also said that "if we (BHP) need to declare force majeure we'll make the appropriate announcement."

It was not clear for how long the mine can produce cathodes during the strike, although the spokesman said earlier the mine has "maintained production and leaching activities at a level that will ensure process and asset integrity."

Union president Andres Ramirez told Reuters workers will keep pressing their demands for a bigger share of profits as copper prices recover.

"The company hasn't called yet ... it's trying to play with the workers to create uncertainty," said Ramirez, who was camping outside the mine located in Chile's Atacama desert. "We will continue to fight for our rights."

Most of the 560 union workers at Spence downed tools on Tuesday after rejecting BHP's final collective contract offer to raise salaries 4 percent. The strike forced the company to halt the mining of the ore that feeds the mine's solvent extraction and electrowinning process, used to produce the cathodes.

Spence produced 164 761 tonnes of copper cathodes in 2008 and any disruptions at its output could further lift copper prices that have seen a steep recovery so far this year.