(Adds Codelco comment)
SANTIAGO, April 12 (Reuters) - Chile, the world's biggest copper producer, faced a fresh threat of labor action in the sector on Wednesday when a union at the large Chuquicamata mine said it had blocked access as a "warning" over planned changes to job opportunities.
Mine owner Codelco said the union's action, which affected the early shift, was "illegal." It ended by 9 a.m. and the impact on production was being evaluated, it said in a statement.
Earlier, Codelco said output had been unaffected as workers from the previous shift had continued to keep operations going.
The state-run company signed new 27-month contracts with its six Chuquicamata unions in December after relatively rapid talks.
However, union no.1 said Wednesday that the contracts were not being fulfilled, as Codelco was making changes that were reducing the possibility of promotion.
"They are cutting off the careers of Chuqui workers, particularly the youngest," union leader Jaime Graz told Reuters. "This is a warning action until they commit to comply with what was agreed."
Codelco said it was complying with the agreed contract terms. "It is not possible to open up and hire for positions that are not needed by the business," it said, adding that it was inviting "constructive dialogue."
The union's move came less than a month after the end of a six-week strike at BHP Billiton's massive Escondida copper mine, which was the longest strike in the country's mining history and which ended without a real resolution.
Chuquicamata, a century-old pit which is being expanded underground, produced 302,000 tonnes of copper last year, out of world no.2 copper miner Codelco's 1.8 million tonnes total. (Reporting by Fabian Cambero and Rosalba O'Brien; Editing by Chizu Nomiyama and Frances Kerry)