(New throughout, adds union comments, background)
SANTIAGO, March 16 (Reuters) - The striking union at Escondida mine in Chile, the world's biggest copper mine, said on Thursday it would return to the negotiating table if the company gave a written guarantee that it would only discuss the union's three key demands.
Escondida did not immediately respond to requests for comment.
During the strike, which started on Feb. 9, Escondida's 2,500-member union has repeatedly said it has three non-negotiable demands mine owner BHP Billiton must commit to before workers return to talks.
It wants Escondida not to trim benefits in its existing contract, not make shift patterns more taxing, and to offer the same benefits to new workers as those already at the mine.
The 35-day strike is the longest in Escondida's history, driving up global copper prices on expectation of tighter supply, and leading economists to increasingly forecast a gloomy quarter for Chile, the world's top copper exporter.
Government-mediated talks failed in February after a few hours, and two invitations for talks from the company over the last week were rejected by the union, citing a failure to mention its key demands.
"The union directorate has decided to give the company another opportunity to rectify its opposition on the three base points that are preventing us arriving at an agreement," the union said in a statement. (Reporting by Fabian Cambero, Writing by Rosalba O'Brien; editing by Diane Craft and David Gregorio)