(Adds comment from Codelco)
SANTIAGO, July 30 (Reuters) - Workers at Codelco's Chuquicamata copper mine in Chile, the state miner's second largest by output, walked off the job on Monday morning and blocked access to the mine, union leaders said, in a move criticized as "illegal" by Codelco.
Chuquicamata Union No. 1, 2, and 3, as well as Antofagasta Union No. 1 said they were protesting the "unjustified layoff" of two workers, according to an internal document seen by Reuters and the union officials.
"All of the unions have ceased operations," said Liliana Ugarte, president of Union No. 2 at Chuquicamata. "How long the walk-off lasts depends on how negotiations go with management."
Codelco, the world's top copper miner, said it was open to further talks.
"It's imperative that we continue production at the mine. This unfortunate and illegal action by (union) leaders affects... us all," the company said in the statement.
The walk-off comes as workers at BHP's Escondida mine, Chile's biggest by output, last week rejected the company's final contract offer, raising the specter of labor action at two of the largest copper mines in Chile, the world's top copper producer.
Unions at Codelco's Chuquicamata have protested for months over plans to transform the century-old open pit into an underground mine.
Workers say Codelco's plans to overhaul the operation - a key part of its $39 billion, 10-year drive to update its aging deposits - failed to address the concerns of the division's nearly 5,664 workers.
Codelco signed 27-month contracts in December 2016 with six Chuquicamata unions after relatively rapid talks, but the labor situation has worsened recently as the mine overhaul encounters delays for technical reasons and rising costs.
Chuquicamata produced 330,900 tonnes of copper in 2017, out of Codelco's total of 1.734 million tonnes. (Reporting by Antonio de la Jara Writing by Dave Sherwood Editing by David Gregorio, Frances Kerry and Jonathan Oatis)