NEW YORK - Subscribers to Amazon.com's Prime service can now get strawberries, milk, meats and frozen dinners on the same day they order them as the company expands its fast-delivery option to perishable food items, Amazon said on Wednesday.

It is the latest move by Amazon to compete with delivery services offered by Walmart+ and Instacart.

The new same-day delivery service is free for Prime members, who pay $14.99 monthly or $139 annually, for orders over $25. Shoppers without a Prime membership have to pay a $12.99 fee, regardless of order size, to use the new service. Walmart's membership service, Walmart+, costs $98 a year and offers same-day delivery in under three hours - with some orders arriving in as little as 30 minutes.

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As of Wednesday, Amazon shoppers in more than 1,000 US cities - including Phoenix; Raleigh, North Carolina; and Tampa, Florida - are able to obtain same-day delivery of perishable food items. The company plans to expand the service to 2,300 cities by the end of the year.

Previously, Prime subscribers' perishable grocery orders were fulfilled through Amazon Fresh or Whole Foods Market, but they had to pay an additional $9.99 per month on their subscription to receive free deliveries on grocery orders that totaled more than $35, according to the company's website. Now the additional service will complete orders through its same-day delivery logistics network.

"This marks a major expansion for Amazon’s digital grocery service, largely because it's being offered to its massive Prime member base at no additional cost," said Blake Droesch, analyst with eMarketer. "Notably, Amazon has lowered the minimum order threshold to just $25 - a move that directly threatens Instacart by enabling customers to use Amazon for quick, one-off purchases, a core part of Instacart's value proposition."

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Brian Mulberry, portfolio manager at Zacks Investment Management, said the new offer is likely to increase Prime's membership, but added that ensuring that the service is high-quality will be critical for growth.

The product update today decreases the barrier to entry for Prime and non-Prime users to buy groceries from Amazon and will likely pressure Uber Eats and DoorDash's margins, said Stephen Ju, analyst at UBS.

Amazon shares were up 1.4 percent on Wednesday afternoon while shares of competitors in the grocery-delivery category lost ground. Shares of Instacart, also known as Maplebear, were down 12.4 percent, while DoorDash and Kroger stocks were down 4.8 percent and 4.2 percent respectively. Walmart shares were down 2.3 percent and shares of Uber, parent of Uber Eats, were down 0.8 percent.

Amazon announced in June it is investing $4 billion to bring same-day and next-day delivery services to more than 4,000 rural US communities by the end of the year.

In May, Walmart said it will soon be able to offer delivery within three hours to 95 percent of the US population, and that faster delivery speeds are helping drive its business.

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Amazon's acceleration into perishable food delivery could strain small, independent grocers across the US, said Chedly Louis, vice president of corporate finance at credit rating agency Moody’s Ratings.

Amazon shoppers "can order milk alongside electronics ... and check out with one cart and have everything delivered to their doorstep within hours," Doug Herrington, CEO of Worldwide Amazon Stores, said in a press release.

While the company is focusing on its US grocery experience, it is facing hurdles in the UK with its suppliers, a study by British grocery regulator the Groceries Code Adjudicator found.