U.S. officials are investigating whether American Express misrepresented its foreign exchange pricing to business clients, a person familiar with the matter told AFP Wednesday.

The probe by the Federal Bureau of Investigation is at an early stage of, said the source, confirming a report in the Wall Street Journal.

The Journal said AmEx recruited corporate clients with offers of low foreign exchange rates, but then raised the rates without warning, boosting company revenues and commissions.

This practice occurred at least since 2004, the Journal reported.

An AmEx spokeswoman said the company believes its transactions "are completed and reported in a fair and transparent manner at the rates our customers have authorized," but that it takes the allegations "very seriously."

"We are conducting a review with an external party in order to determine whether all of our standards are being met," the spokeswoman said.

"If we find that we fell short of the mark, we will fix the problems and take appropriate actions to make sure it doesn't recur."

Shares of AmEx (AXP) closed down 1.2 percent at $105.68, dropping after the Wall Street Journal article was published.