The e-commerce giant's year-over-year unit sales, a closely watched measure of how many items Amazon has sold, has slowed considerably over the last two years. Unit sales growth was 25 percent in the fourth quarter.

During a call with analysts to discuss the company's first-quarter results, Chief Financial Officer Tom Szkutak also said Amazon now has 244 million active customer accounts.

(Reporting by Deepa Seetharaman; Editing by Bernard Orr)