NEW YORK (AP) — Walmart is expanding its same-day online grocery delivery service to more than 40 percent of U.S. households, or 100 metro areas, by year-end as it tries to keep pace with online leader Amazon.com.

The service is currently available in six markets.

Tom Ward, vice president of Walmart's digital operations, says the retail giant is powering the expansion of its same-day delivery service using its online grocery pickup program. That service uses personal shoppers to select items and then take them to shoppers' cars parked at the curb.

So far, Walmart offers curbside grocery pickup at 1,200 stores and plans to accelerate the rollout to 2,200 by year-end. It's currently using 18,000 personal shoppers.

Walmart says it will continue to use of ride-hailing services like Uber to deliver the goods to shoppers' homes as it expands its service.

Shoppers pay a flat fee of $9.95 but are required to spend at least $30 per order.

Walmart Inc. has also been testing a service using its U.S. store workers to drop off general merchandise like toys and bedding to customers' homes after they finish work.

Ward noted that Walmart is learning from its delivery services in such countries as China and the United Kingdom.

Amazon's purchase of Whole Foods Market last year has raised the stakes in the highly competitive grocery delivery wars. Amazon recently added free two-hour Whole Foods delivery to six cities, including Atlanta, Dallas and Cincinnati for its Prime members who pay $99 a year.

Meanwhile, grocery startup Instacart has been expanding its roster of clients including B.J.'s and Kroger.

And Target, through its acquisition of grocery delivery startup Shipt last year, is expanding same-day delivery of such items as groceries and electronics to nearly every major market by the holiday shopping season.

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