NEW YORK, United States — Wal-Mart Stores Inc said full-year sales would be flat due to the stronger-than-anticipated impact of the dollar's strength, and that investment in technology and employees would pressure earnings next year, sending its shares down 10 percent.

Shares of the company, which also announced a $20 billion share buyback, fell to a more-than three-year low of $60.18 on Wednesday, wiping out about $21 billion in market value.

The stock was on track for its worst one-day performance in more than 17 years.

"We can deliver stronger financial performance in the short-term simply by running our core business better but that won’t be enough," Chief Executive Doug McMillon said at an investor meeting in New York.

Wal-Mart had previously forecast net sales growth of 1-2 percent for the current year ending January.

The company also said it now expected earnings per share to decline by 6-12 percent next fiscal year.

"The guidance is very disappointing," Edward Jones analyst Brian Yarbrough said. "What if these investments don’t lead to better sales. That's the biggest question."

McMillon said earlier on CNBC that the strong dollar would likely reduce the company's full-year revenue by $15 billion.

He also said Wal-Mart would add 3,000 department managers in the United States this year to improve its curbside grocery pickup service ahead of the holiday season.

"The opportunity to leverage stores for pickup is a huge one... we are announcing 11 more markets now and will be up to more 20 markets in the U.S. by end of this year," he said.

Wal-Mart said last month it would expand free grocery pickup service as it seeks to capitalize on its network of physical stores amid growing competition with Amazon.com and others investing in home delivery.

Wal-Mart has been grappling with sluggish sales, leading investor to seek significant changes.

The company announced a new chief financial officer and appointed a chief merchant last week.

By Sruthi Ramakrishnan, Nathan Layne; editors: Anil D'Silva, Ted Kerr.