(Adds comments from union spokesman)
By Fabian Cambero
ANTOFAGASTA, Chile, Feb 9 (Reuters) - Workers at BHP Billiton's BHP.AX BLT.L Escondida in Chile, the globe's biggest copper mine, walked off the job on Thursday, their union said, marking the start of a strike that threatens to disrupt the international supply of the metal.
No miners were aboard transport normally filled with workers slated to begin the morning shift, it said. Striking workers also blocked roads at the Coloso port, where hundreds of thousands of tonnes of copper are shipped annually.
"The buses are empty, there are no workers in them" to replace the night shift, union spokesman Carlos Allendes told Reuters. "The strike is now in effect."
BHP Billiton spokesmen were not immediately available for comment.
BHP has said it would halt production during the strike because it could not guarantee the safety of the 80 workers the government had authorized to perform "critical duties," such as equipment upkeep and adhering to environmental protocols.
The union has warned that a strike could be lengthy, and workers camped near the mine have stockpiled rations and supplies to survive the Chilean high desert's scorching sun and frigid nights.
"We are united and strong to make sure this is a success considering the measures the company has taken against its workers; we are fighting for our families and our future," said Claudio Perez, plant worker at the Coloso port.
The strike at Escondida, which produced 6 percent of the world's copper in 2015, follows weeks of fractious negotiations which have sent copper prices higher.