The union on strike at BHP Billiton's Escondida copper mine in Chile again rejected an invitation by the company to return to negotiations as worker demands were not adequately addressed, leaving the parties without an obvious path forward.

In a statement late Monday, the 2500-member Escondida union, which has been on strike since February 9, said BHP did not sufficiently address workers' demands in its latest invitation sent to the union earlier on Monday.

Benchmark copper on the London Metal Exchange was untraded in official rings, but was bid 0.5 per cent lower at $US5770 a tonne. The metal used in power and construction is up more than 2 per cent since hitting a two-month low at $US5652 last week.

The union demands include that BHP agrees not to trim benefits in the existing contract, that shift patterns should not be made more taxing for workers, and new workers be offered the same benefits as those already employed at the mine.

"After revising its content, the union members have decided that unfortunately this response presents a series of inconsistencies and falsehoods," the statement said.

"The union will maintain the strike for an indefinite amount of time," it added.

It was the third failed attempt to restart dialogue during the strike, which has pushed global copper prices higher due to supply concerns.

On Friday, BHP, which could not be immediately reached for comment, invited the union to return to negotiations. The union rejected that invitation, saying it did not respect workers' core non-negotiable demands.

Escondida, majority owned by BHP, produced slightly more than one million tonnes of copper in 2016, making it the world's largest copper mine.

Rio Tinto and Japanese companies such as Mitsubishi hold minority interests in the mine.