WASHINGTON, Dec. 28 (Xinhua) -- Citigroup was fined by a U.S. regulator 5.5 million U.S. dollars and required to pay at least 6 million in compensation to customers for displaying inaccurate stock ratings.

The Financial Industry Regulatory Authority (FINRA), an independent, non-governmental regulator for all securities firms doing business with the public in the U.S., said the New York based bank displayed inaccurate research ratings for numerous equity securities during a nearly five-year period.

From February 2011 through December 2015, Citigroup displayed to its brokers, retail customers and supervisors inaccurate research ratings for more than 1,800 equity securities, said FINRA.

Because of errors in the electronic feed of ratings data that the firm provided to its clearing firm, the firm either displayed the wrong rating for some covered securities, displayed ratings for other securities that Citigroup did not cover or failed to display ratings for securities that it in fact rated, according to FINRA.

The firm's actual research reports, however, were not affected by these errors, the regulator added.

"The display and use of incomplete and inaccurate research ratings can have widespread, adverse consequences to customers. Even when such inaccuracies are caused by technology problems, firms should react quickly to address those errors," said Susan Schroeder, FINRA Executive Vice President and Head of Enforcement.

Citigroup did not admit or deny wrongdoing, but the sanctions reflected its cooperation, FINRA said.