SANTIAGO – Striking workers at BHP Billiton's Escondida copper mine in Chile, the world's largest, said they will hold negotiations with the company on Monday, but the union imposed a condition that it will only discuss workers' three main demands.

Union members have also given union leaders authority to walk away from negotiations and force a temporary, 18-month contract, as permitted under Chilean law, it said.

"The position of the workers...will be to only discuss the three points with the company that have stopped negotiations during the 40 days of the strike," the union said in a statement released to Reuters. The meeting will take place on Monday evening.

BHP could not be immediately reached for comment.

The 2 500-member union at Escondida has been on strike since February 9, and production has been stopped since then, sending global copper prices higher amid supply concerns.

Workers have three core demands. First, benefits in the existing contract must not be reduced. Second, work shifts may not be made more taxing. And third, new workers must receive the same benefits as those already at the mine.

On Thursday, the union invited the company to return to the negotiating table, on the condition that BHP give a written guarantee that talks would focus on the trio of demands.

The company agreed but was ambiguous about the demand to discuss only the union's key issues. With the Monday decision, union leaders decided to meet, with conditions attached.

Union leaders also said in the statement on Monday that they had received approval from the rank-and-file to invoke Article 369 of Chile's labour code, if the leaders deem appropriate.

That article would legally halt the current negotiation process and maintain the benefits of the current contract for 18 months, postponing collective wage talks.

Such an agreement is not often invoked by workers, as it delays the one-time bonus typically given to miners when contracts are inked.

However, the union said delaying the wage talks would allow the next round of negotiations to occur under Chile's new labour code, which is set to go into effect in April, giving the union more power.

"This option allows us to avoid the plans of the company to reduce benefits and remunerations," the union said.

Escondida produced slightly over one-million tonnes of copper in 2016. Rio Tinto and Japanese companies including Mitsubishi have minority interests in the mine.