The Union No1 went on strike at the BHP (AU:BHP) majority-owned mine in February 2017 for 44 days, then exercised its right to extend the existing contract for 18 months.

Sindicato No 1 confirmed it had received a formal invitation to start talks ahead of the mid-year deadline.

In a translated statement, it said it had been asked for a negotiation period of three weeks but said committing to a deadline was not appropriate.

It also noted the invitation was made in the context of a substantial improvement in the copper price, new labour legislation and a significant increase in the company's production.

BHP CEO Andrew Mackenzie had told analysts in a briefing last month that the company fully intended to talk to the union about the possibility of getting a settlement ahead of the point it was possible for a legal strike.

Analysts have said the copper market is at risk of disruption this year, with the upcoming labour negotiations in Chile and Peru, while some workers at First Quantum Minerals' (CN:FM) 90%-owned Cobre Panama copper-gold project have gone on strike this month and the possibility of a strike looms at Antofagasta's (LN:ANTO) Los Pelambres mine in Chile.