BUZZ-U.S. ad firms slip after London rival WPP's weak results

Shares of ad firms Omnicom OMC.N and Interpublic Group IPG.N fall 2.6% and 0.9%, respectively

London rival WPP WPP.L reports worse-than-expected decline in organic revenue hurt by weak client spending

WPP CEO Mark Read said it was a "tough market out there," amid pressure in British market, the U.S. and China

OMC and IPG are set to complete a $13.25 billion all-share merger this year that would make them the world's biggest ad group

IPG down 14.3% in the past 12 months, OMC has lost 6% in the same period