June 9 (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 ( OMC ), WPP ( WPP ), Dentsu ( DNTUF ), Interpublic

Group and Publicis Groupe are among the

agencies that the FTC sent letters to on Monday.

Reuters could not immediately verify the report.