* Union says BHP's initial proposal "insufficient"
* Strike seen unlikely as law prohibits early walkout
By Anthony Esposito
SANTIAGO, Dec 26 (Reuters) - Workers at the world's biggest copper mine, Chile's Escondida, have rejected an early pay offer made by mine controller BHP Billiton, a union leader said on Wednesday.
The risk of a strike appears remote, however, because it is illegal to go on strike in Chile before official pay talks have begun. Collective contract negotiations at Escondida are due to take place after the current contract expires in June 2013.
"Workers rejected the company's proposal as they considered it insufficient," union secretary Marcelo Tapia told Reuters.
"This was an invitation by the company to have early collective negotiations, which should actually happen in July 2013. We're going to officially inform them on Dec. 28 of workers' rejection of their proposal," he added.
BHP was not immediately available to comment. No details were available on the company's proposal or the union's demands.
If no early wage offer is accepted, workers and the company will sit down for wage talks as scheduled next year.
Escondida's union stunned the copper market last year by staging a two-week strike, sending the mine's output tumbling and raising the specter of an increase in labor action.
BHP and Rio Tinto, which owns 30 percent of the mine, have approved plans for a $4.5 billion expansion of Escondida to boost output.
Escondida's third-quarter output surged 72.4 percent from a year earlier to 253,800 tonnes, boosted by better ore grades and a low base of comparison from the year-ago quarter. Output in the January-September period was 787,000 tonnes, up 31.6 percent from a year earlier.
Earlier this month, workers at the giant Chuquicamata mine of Chile's state-run copper firm Codelco accepted a wage offer with the company.