Staples said Thursday it will close 225 stores by the end of 2015 amid falling fourth-quarter revenue as sales increasingly shift online.

In morning trading, its stock is tanking, down 13.2% to $11.63.

"With nearly half our sales generated online today, we're meeting the changing needs of business customers and taking aggressive action to reduce costs and improve efficiency," Staples CEO Ron Sargent said.

For the fourth quarter, total company sales fell 10.6% to $5.9 billion vs. the year-ago period, the nation's No. 1 office supply chain said. Profit fell to $1.5 billion from $1.7 billion.

The store closings will affect nearly 15% of the company's 1,500 U.S. outlets and are part of a plan to save $500 million in costs by the end of next year.