Workers who deliver packages from seven Amazon facilities across the country went on strike Thursday morning, according to the International Brotherhood of Teamsters, the union that represents them.

The Teamsters said thousands of workers had struck, but it was unclear how many workers were participating in the action. Amazon said it expected the seven distribution centers to operate normally.

The workers are employees of companies that Amazon uses to deliver packagers to customers. Amazon has said it has no obligation to bargain with the drivers, because they are not its employees. But the union and the workers said that Amazon ultimately controls their working conditions and was therefore obligated to bargain with them.

The National Labor Relations Board has investigated the cases and issued at least one complaint finding the workers to be Amazon employees and accusing the company of breaking the law by failing to bargain with them.

This is a developing story. Check back for updates.