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 people were participating in the action. Amazon said it expected the seven delivery hubs to operate normally.

The drivers are employees of companies that Amazon uses to deliver packages 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 some of the cases and issued at least one complaint finding the drivers to be Amazon employees and accusing the company of breaking the law by failing to bargain with them.

The Teamsters said in a statement that workers at other Amazon warehouses were prepared to join the strike. The largest group at Amazon represented by the union works at a Staten Island warehouse in New York known as JFK8, which employs more than 5,000 people. Employees at the warehouse voted to unionize in 2022, but the company has yet to bargain with them and is challenging the election outcome.

Workers involved in the strike say it could extend into early next week, perhaps into Christmas, but it’s unclear how big an impact the walkout will have on Amazon’s holiday deliveries.

The company has hundreds of delivery hubs across the country, and 10 or more in some of the largest urban areas. It has said that if one delivery hub experiences delays or driver shortages, the company can seamlessly shift packages to another hub nearby.

Former delivery hub managers say these methods have helped the company reliably avoid disruptions in the past, although the ability to navigate a strike is likely to diminish during the holiday season, when the network has less spare capacity.

At the delivery hubs, Amazon generally hires multiple contracting firms to take packages to customers. The Teamsters has organized the drivers who work there by asking them to sign authorization cards. The union typically proceeds by trying to organize individual contracting firms rather than an entire complex at once. In most cases, the owners of the contracting firms have yet to recognize the union and there have not been elections.

“What you see here are almost entirely outsiders — not Amazon employees or partners — and the suggestion otherwise is just another lie from the Teamsters,” Amazon said in a statement, referring to people picketing outside the delivery hubs. “The truth is that they were unable to get enough support from our employees and partners and have brought in outsiders to come and harass and intimidate our team.”

The Teamsters had set a deadline of Dec. 15 for Amazon to begin negotiating with the drivers and warehouse employees. It initiated the strike after Amazon failed to do so.

“If your package is delayed during the holidays, you can blame Amazon’s insatiable greed,” Sean O’Brien, the Teamsters president, said in a statement. “We gave Amazon a clear deadline to come to the table and do right by our members. They ignored it.”

Three of the seven hubs are in Southern California, and the other four are in Queens, New York; Atlanta; San Francisco; and Skokie, Illinois, near Chicago.