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Production was interrupted at Colombia's Barrancabermeja refinery for a fourth straight day on Monday as union members demanded improved conditions for contracted workers.
In a statement, the USO union accused state oil company Ecopetrol of refusing to negotiate and threatened to extend stoppages to production fields.
Ecopetrol said dialogue hinged on an end to alleged violence by contractors against other workers.
Local media reported that protesters have so far impeded the production of more than 5,000b of oil derivatives as demonstrators block access to the complex for more than 2,000 workers.
Barrancabermeja, in central Santander department, is Colombia's largest refinery with installed processing capacity of 250,000b/d.
Last week, Ecopetrol said it suspended 27 service contracts for two oil and gas fields in central Meta department amid ongoing violence with disaffected workers. USO said the unrest owed to a breach of labor agreements.