Wal-Mart Stores Inc., already reeling from a sales slump and higher labor costs, suffered its worst intraday stock decline in more than 15 years after predicting that profit will decline during the next fiscal year.

Wal-Mart Stores Chief Executive Doug McMillon also said the strong dollar would likely reduce the company's full-year revenue by $15 billion.

Earnings will decrease 6 percent to 12 percent in fiscal 2017, which ends in January of that year, the Bentonville, Arkansas-based company said at a conference on Wednesday. Analysts had estimated a gain of 4 percent on average according to data compiled by Bloomberg. The retail chain also said its board has authorized $20 billion in stock buybacks.

Wal-Mart shares fell as much as 9.8 percent to $60.18 in New York, the biggest drop since February 2000. The stock was already down 22 percent this year before the decline.

Wal-Mart, the world’s largest retailer, is trying to win back customers by improving the shopping experience, expanding its online grocery pickup service, and opening more small-format stores, called Neighborhood Markets. It has also been spending $1 billion to improve its website and opening new distribution centers, aiming to speed the delivery of online orders. Investors, though, have been skeptical that the changes will reignite growth.

"There is no way they can continue to grow, they are just too big," Ivan Feinseth, chief investment officer at Tigress Financial Partners, said before Wednesday’s forecast. "They do $500 billion worth of revenue — how are you going to grow that?"

The world's largest retailer, which McMillon told CNBC gets about a third of revenue and a quarter of profit from outside the United States, reported revenue of $485.7 billion last year.

The company is also adding 3,000 department managers in the United States this year to improve its curbside grocery pickup service ahead of the holiday season, McMillon said.

"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, Reuters reported.

Wal-Mart has been grappling with sluggish sales, leading investor to seek significant changes at the company. It announced a new chief financial officer and appointed a chief merchant last week.