Workers at seven Amazon facilities went on strike Thursday, an effort by the Teamsters union to pressure the e-commerce company for a labor agreement during a key shopping period.

The Teamsters said the workers joined picket lines after Amazon ignored a Sunday deadline the union set for contract negotiations.

The International Brotherhood of Teamsters says it represents 10,000 workers at 10 Amazon facilities, a small portion of the 800,000 workers employed in the company’s U.S. warehouses.

The strikes Thursday are taking place at seven delivery stations, which are run by contractors who drop off packages to customers every day. They include three locations in Southern California, and one each in Atlanta, New York City 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 declined 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.