Wal-Mart Chief Executive Doug McMillon said a $1.5 billion investment in wages and training, including raising the minimum store wage to $10 an hour from $9, were needed to improve customer service and would account for three-quarters of the expected 6 percent to 12 percent drop in earnings per share next year.

Wal-Mart also announced a $20 billion share buyback but the drop in its share price wiped out close to the same amount in market value, and the 10 percent drop was the worst one-day percentage performance since January 1988.

The world's largest retailer by revenue said it would invest several billion dollars to lower prices over the next three years. That sparked worries of a price war, and shares of rivals including Target and Home Depot also fell.