NEW YORK (AP) - Walmart is rolling out next-day delivery on its most popular items, raising the stakes in the retail shipping wars.

The nation's largest retailer says Tuesday it's been building its own network of more efficient e-commerce distribution centers to enable the faster delivery. The move comes just two weeks after online behemoth Amazon said it's upgrading its free shipping option to Prime members who pay $119 a year to one-day delivery from two-day delivery.

Analysts say Amazon needed to cut the delivery time in half to make its membership more attractive since other retailers like Walmart had offered free two-day deliveries without any membership.

Walmart's stepped-up efforts will only increase pressure on other rivals that are already investing in millions of dollars to shorten the delivery window.

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