The United Steelworkers said BP engaged in unfair labor practices by locking out nearly 800 workers at its Whiting, Indiana, oil refinery and urged the company to resume contract talks.
"BP's decision to lock out these skilled workers is unacceptable and unlawful," said USW International President Roxanne Brown.
The union said it remains ready to negotiate.
Last week, the British oil major barred the workers from entering the facility, following months of stalled contract negotiations.
BP locked out the workers after the Steelworkers twice rejected its "last, best and final" offer.
"While we are disappointed in the union’s rejection, BP has bargained, and will continue to bargain, in good faith," a company spokesperson said, adding that the lockout can be resolved as soon as the Steelworkers agree to the proposal on the table.
BP said it does not expect the lockout to disrupt production.
