NEW YORK — Amazon, which opened a cashier-less store in Seattle this year, plans more locations in San Francisco and Chicago.

The company confirmed Monday that Amazon Go stores would open in the two cities, but didn't say when. The store openings were first reported by the San Francisco Chronicle and The Seattle Times.

Shoppers enter the stores by scanning a smartphone app. Once inside, they can take soda, salads or anything else off its shelves and just walk out. Amazon's technology, including overhead cameras, keeps track of what they buy and charges customers after they leave.

Opening more stores is yet another sign that Amazon is serious about expanding its physical presence. It bought grocer Whole Foods last year, which gave it 470 stores, and has opened more than a dozen bookstores.

The online retailer opened its Amazon Go concept to the public in January, selling milk, potato chips and other items typically found at a convenience store. Amazon employees have been testing the store, which is at the bottom floor of the company's Seattle headquarters, for about a year.

The public opening is another sign that Amazon is serious about expanding its physical presence. It has opened more than a dozen bookstores, taken over space in some Kohl's department stores and bought Whole Foods last year, giving it 470 grocery stores.

But Amazon Go is unlike its other stores. Shoppers enter by scanning the Amazon Go smartphone app at a turnstile. When they pull an item of the shelf, it's added to their virtual cart. If the item is placed back on the shelf, it is removed from the virtual cart. Shoppers are charged when they leave the store.

The company says it uses computer vision, machine learning algorithms and sensors to figure out what people are grabbing off its store shelves.

The store is not without employees — Amazon says there will be people there making food, stocking shelves and helping customers. On Monday, workers were on hand to help shoppers find and download the Amazon Go app on their phones.

Amazon says families can shop together with just one phone scanning everyone in. Anything they grab from the shelf will also be added to the tab of the person who signed them in. But don't help out strangers: Amazon warns that grabbing an item from the shelf for someone else means you'll be charged for it.

At about 1,800 square feet, the store will also sell ready-to-eat breakfasts, lunches and dinners. Items from the Whole Foods 365 brand are also stocked, such as cookies, popcorn and dried fruit.

The company had announced the Amazon Go store in December 2016 and said it would open by early 2017, but it delayed the debut while it worked on the technology and company employees tested it out.