New YorkCNN — 

About 18,000 Teamsters are set to strike 56 Costco stores across six states early Saturday unless an 11th-hour agreement prevents what would be the largest retail strike in US history.

Talks resumed between the two sides Thursday, but there have been no updates on the negotiations. In addition to disagreements on wages and benefits, the union said Costco had rejected proposals dealing with seniority pay, paid family leave, bereavement policies, sick time, and safeguards against surveillance.

Teamsters members at Costco make up 8% of the 219,000 employees at 616 US stores, according to company filings. But a strike by that many workers would be significant for the largely nonunion sector. Labor Department statistics show that less than 5% of retail workers are represented by unions. Most of those unionized retail workers are in the grocery store sector.

The unionized store locations, which have not been identified by either the union or company, are in California, Maryland, New Jersey, New York, Virginia and Washington. Costco said on Friday that even if there is a strike and unionized workers don’t report to work, “all of our locations will be open for business this weekend,” an indication that it intends to use management and nonunion staff to keep stores operating.

Meanwhile, while the Teamsters only represent workers at a small fraction of Costco stores overall, the union is prepared to set up picket lines at an undisclosed number of nonunion Costco stores nationwide, even if workers at those stores remain on the job. Such a move could greatly increase the economic pressure on the company. While there are only 18,000 Teamsters at Costco, the union has more than 1 million US members it can call on for those other picket lines.