MEXICO CITY (Reuters) - Mexican election results indicate President-elect Enrique Pena Nieto's coalition will not have a majority in either house of Congress, electoral sources said on Monday.

Passing reforms could prove an uphill battle and he may need to negotiate with opponents including the conservative National Action Party (PAN), whose own reform agenda was stymied by Pena Nieto's Institutional Revolutionary Party (PRI).

(Reporting by Tomas Sarmiento, Miguel Angel Gutierrez and Lizbeth Diaz)