Amazon just fired the latest shot against Walmart in the free shipping wars

Amazon said Monday that its Prime free one-day shipping offer now applies to more than 10 million items.

This means that Amazon's one-day shipping program is already much larger than Walmart's.

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Amazon said Monday that it is now offering free one-day shipping for more than 10 million items to Prime members across the US.

This means that Amazon's one-day shipping offering is already much larger than the next-day shipping program that Walmart launched three weeks ago.

Walmart's next-day shipping offer, by comparison, will initially be available only on a selection of up to 220,000 items, which will vary regionally based on customers' ordering habits.

Read more: Walmart launches salvo to blow Amazon out of the water with its own free, next-day delivery

Walmart launched one-day shipping in several cities last month, and plans to make it available to 75% of US consumers by the end of the year. Walmart said it plans to grow the assortment of items available for next-day shipping over time. To access to the free next-day shipping, customers must make a minimum purchase of $35.

Amazon said in April that it would shorten its two-day Prime shipping promise to one day. The service is available to members of Amazon's Prime program, which costs $119 annually, and requires no minimum purchase amount.