NEW YORK, United States — Wal-Mart Stores Inc., racing to fend off Amazon.com Inc. and a fresh attack from European grocery discounters, gave a lukewarm earnings forecast for the third quarter, a sign that heavy spending aimed at maintaining its edge is taking a toll.

The shares slid in early trading Thursday after the retail giant said that profit will be 90 cents to 98 cents a share in the period. Analysts had projected a number at the top of that range.

The outlook tempered enthusiasm after Wal-Mart posted its best grocery sales growth in five years last second quarter, helped by the end of a record-setting bout of food deflation. The grocery business accounts for more than half of Wal-Mart’s revenue, but it’s under attack like never before. Most notably, Amazon is acquiring organic-food purveyor Whole Foods Market Inc. in a bid to become a national grocery powerhouse.

“We have to take things that are working and lean into them,” Wal-Mart Chief Financial Officer Brett Biggs said in an interview.

The shares declined as much as 2.4 percent to $79.03 in premarket trading after the results were posted. Wal-Mart had been up 17 percent this year through Wednesday’s close.

The downbeat forecast illustrates the high cost of Wal-Mart’s push to catch up to Amazon online. Chief Executive Officer Doug McMillon has channeled more than one-third of the company’s capital spending budget into digital initiatives -- like specialized e-commerce distribution centers -- up from just 20 percent a few years ago.

Profit margins on online sales are narrower than those for in-store sales, due to the costs of fulfillment. Wal-Mart’s operating margins declined in the quarter, according to Moody’s Corp. analyst Charlie O’Shea.

U.S. same-store sales rose 1.8 percent in the second quarter, matching the gain projected by analysts. Excluding some items, earnings amounted to $1.08 a share in the period, which ended on July 31. Analysts estimated $1.07. The Bentonville, Arkansas-based company raised the lower end of its full-year earnings guidance to $4.30 a share, with the upper end remaining at $4.40.

The question now is whether Wal-Mart can battle Amazon while also keeping Aldi and Lidl at bay. The German no-frills chains are expanding into the U.S., targeting the same lower-income shoppers that Wal-Mart depends on most.

Though costly, Wal-Mart’s investments in e-commerce are helping boost its top line. The U.S. online division saw gross merchandise volume — a measure of all the goods it sells — increase 67 percent in the second quarter. Total revenue climbed 2.1 percent to $123.4 billion.

Wal-Mart also is facing a renewed threat from longtime brick-and-mortar rival Target Corp. That company boosted its annual forecast earlier this week after improving both its online sales and customer traffic. CEO Brian Cornell aims to refurbish stores, open more small locations in cities and speed the delivery of online orders.

Heavy discounting should continue this quarter, “as Wal-Mart and Amazon continue their battle over market share,” O’Shea said in a note.

“As Wal-Mart is a key player across most back-to-school/back-to-college product categories,” he said, “we would expect further promotional activity.”

By Matthew Boyle; Editors: Caroline Salas Gage, Nick Turner, Molly Schuetz