Walmart is expanding its online grocery delivery service to more than 100 metro areas in the United States as it tries to keep pace in an increasingly expensive turf war with Amazon, Target and other retailers over the lucrative e-commerce market.

On Wednesday, Walmart said its delivery service, currently offered in six cities and surrounding areas, would be available to more than 40 percent of U.S. households by the end of the year. However, the company did not announce the new metro areas where it would add service.

Some orders would be delivered on the day they are placed, the company said. Walmart plans to use 800 of its own stores to fulfill customer requests, and partner with Uber and other driving services for delivery.

"We're saving customers time by leveraging new technology, and connecting all the parts of our business into a single seamless shopping experience: great stores, easy pickup, fast delivery and apps and websites that are simple to use," Greg Foran, chief executive of Walmart U.S., said in a statement.

Walmart charges $9.95 for its delivery service and requires a minimum order of $30.

The company's newest challenge to Amazon targets two industries that are increasingly appealing to retailers: e-commerce and grocery. Ever since Amazon rattled rivals in June by buying the upscale grocery chain Whole Foods for $13.4 billion, other retailers have been racing to provide home delivery of groceries bought online.

In October, Costco introduced a two-day delivery option for dry groceries and a same-day alternative for fresh goods through Instacart, a delivery service. Target said in December that it would purchase the online same-day delivery service Shipt for $550 million in cash.

Earlier this month, Amazon said it would offer same-day delivery of groceries from Whole Foods in six cities. On Monday, Kroger followed suit with an announcement that it would expand the number of cities eligible for home delivery of groceries through Instacart.

But a grocery delivery service could be difficult to implement and market to shoppers, experts said.

"There is a lot of experimenting going on as everyone tries to figure out that last-mile delivery — it's a tough economic equation to make work," said Mike Knemeyer, a professor of logistics at Ohio State University. "But if you can, you'll have a big head start on the others, and you'll end up making money not just in groceries but on all of the things that you sell."