(Reuters) -The U.S. Federal Trade Commission has sought information from some of the world's top advertising firms as part of a probe into whether advertising and advocacy groups violated antitrust laws by coordinating boycotts of certain sites, the Wall Street Journal reported.

Omnicom, WPP, Dentsu, Interpublic Group and Publicis Groupe are among the agencies that the FTC sent letters to on Monday.

Reuters could not immediately verify the report.