The Teamsters said the workers, who voted to authorize strikes in recent days, joined picket lines after Amazon ignored a Sunday deadline the union set for contract negotiations. The union called it the largest strike against the company in U.S. history, although Amazon said it did not expect the labor action to impact its operations.

The International Brotherhood of Teamsters claims it represents nearly 10,000 workers at 10 Amazon facilities, a small portion of the 800,000 workers employed in the company’s U.S. warehouses. The union hasn’t said how many workers would participate in the strike or how long the walkout would last.

“Amazon is pushing its workers closer to the picket line by failing to show them the respect they have earned,” Teamsters General President Sean O’Brien said in a statement.

The strikes happening Thursday are taking place at seven delivery stations, which are run by contractors who drop off of packages to customers everyday. They include three locations in Southern California, and one each in New York City, Atlanta, Georgia, and Skokie, Illinois, according to the union’s announcement.

The biggest warehouse affiliated with the Teamsters is located in the New York City borough of Staten Island. In 2022, thousands of workers at the warehouse, known as JFK8, voted to be represented by the nascent Amazon Labor Union. Workers then choose to affiliate with the Teamsters this past summer.

The National Labor Relations Board certified that election to unionize, but Amazon has refused to bargain on a contract. In the process, the company has filed a lawsuit challenging the constitutionality of the labor board.

At the other six facilities, employees – including many delivery drivers – unionized with the Teamsters by demonstrating majority support but without holding government-administered elections. Under labor law, companies can recognize unions without elections being held, but the practice is rare, said John Logan, director of labor and employment studies at San Francisco State University.

Amazon workers in more locations are “prepared to join” the fight, the Teamsters said, noting that employees at the Staten Island warehouse and at a company air hub in California also have authorized strikes.

When asked about the strike Thursday, Amazon spokesperson Kelly Nantel said: “What you see here are almost entirely outsiders, not Amazon employees or partners, and the suggestion otherwise is just another lie from the Teamsters.”

“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, which is inappropriate and dangerous,” Nantel said.

Amazon has said it does not consider delivery drivers like the ones on strike to be its employees. Under the company’s business model, the drivers work for third-party businesses, called Delivery Service Partners, who deliver millions of packages daily. Amazon has accused the union, which says it represents some of the drivers, of “intentionally” misleading the public.

“This is another attempt to push a false narrative,” Nantel said.

But the Teamsters have argued Amazon essentially controls everything the drivers do and should be classified as an employer. Some U.S. labor regulators have sided with the union in filings made before the NLRB. In September, Amazon boosted pay for the drivers amid the growing pressure.

Shares of Amazon.com Inc. rose more than 2.4% by midday Thursday.