By Nandita Bose

CHICAGO (Reuters) - Wal-Mart Stores Inc (WMT.N) reported better-than-expected U.S. comparable sales on Thursday, as customers stocked up on food and other supplies ahead of hurricanes and online purchases soared, sending its shares up more than 8.5 percent.

The third quarter marked Wal-Mart's strongest U.S. sales growth since 2009 despite sluggish demand and competition from Amazon.com Inc (AMZN.O) that has hurt brick-and-mortar rivals. The retailer has notched more than three straight years of overall comparable sales growth.

"Momentum in the business is really strong," Wal-Mart Chief Financial Officer Brett Biggs said on an earnings conference call.

Hurricane-related sales contributed 30 to 50 basis points to overall comparable sales, the world's largest retailer said, as Hurricanes Harvey and Irma spurred demand for food and grocery items as well as building materials.

"Food delivered its strongest quarterly performance in six years," Wal-Mart U.S. Chief Executive Greg Foran said.

Wal-Mart has invested in its online business, cut prices and improved its in-store shopping experience by raising worker wages and making stores cleaner and more efficient.

These investments weighed on operating margins and profit, however, which both fell compared to the year-ago quarter.

Online sales soared 50 percent, exceeding growth at other big retailers but coming in below the previous quarter's 60 percent surge.

Those sales added 80 basis points to the quarter's comparable sales gain, boosted by walmart.com, the retailer's online grocery delivery service and the acquisition of e-commerce startup Jet.com last year.

"Wal-Mart's online performance continues to validate its substantial investments in this critical channel," said Moody's retail analyst Charlie O'Shea.

Wal-Mart forecast strong performance in the key holiday quarter and raised its full-year profit forecast. It now expects earnings per share of $4.38 to $4.46 for the fiscal year versus $4.30 to $4.40 previously.

Rival Target Corp (TGT.N) has also slashed prices and invested in its website and delivery, but has been less successful at translating that into sales and profit growth. Its holiday-quarter profit forecast fell short of analysts' expectations on Wednesday, sending shares down 10 percent.

The holiday shopping season can account for as much as 40 percent of a retailers' annual sales. Wal-Mart has said it will double down on incentives and has tripled its online selections.

To compete with Amazon, Wal-Mart will offer free two-day shipping on more than two million products when the order exceeds $35 and discounts for online orders picked up at stores.