American Express stock fell over 2% as it posted higher sales and profit in the second quarter, thanks to higher spending among its credit card members.

The credit-card company on Friday posted a profit of $3.11 billion, or $4.53 a share, in the quarter ended June 30, compared with $2.89 billion, or $4.08 a share, a year earlier. Analysts polled by FactSet expected $4.40 a share.

Revenue net of interest expense rose 10% to $19.64 billion. Analysts surveyed by FactSet had forecast revenue of $19.70 billion.

The increase in sales was primarily driven by higher card member spending, which increased 9%, the highest growth rate in three years.

Card fee revenue also rose, as did net interest income, due to growth in card balances.

"Six months into the year, we're seeing stronger momentum than we expected," Chief Executive Stephen Squeri said.

Amex plans to use the money from this outperformance to invest further in its growth initiatives, including its ecosystem of membership credit cards that come with annual fees and a range of perks. During the second quarter, Amex proposed an acquisition of the European restaurant booking platform TheFork in an effort to expand its dining perks.

Amex's Platinum card, which is one of its cards with the highest annual fee and most perks, is now its fastest growing product in its U.S. consumer business.

Amex is now expecting full-year sales will increase 10%, compared with its previous guidance of 9% to 10% growth. It maintained its outlook for earnings per share of $17.30 to $17.90.