(Reuters) - U.S. authorities are stepping up investigations, including an FBI criminal inquiry, into possible violations by employees of Rupert Murdoch's media empire of a U.S. law banning corrupt payments to foreign officials such as police, law enforcement and corporate sources said.

But U.S. investigators have found little to substantiate allegations of phone hacking inside the United States by Murdoch journalists, the sources added.

The FBI is conducting an investigation into possible criminal violations by Murdoch employees of the U.S. Foreign Corrupt Practices Act (FCPA), a law intended to curb payment of bribes by U.S. companies to foreign officials, a U.S. law enforcement official said.

The U.S. official said that if any law enforcement action was pursued by U.S. authorities against Murdoch employees, it would most likely relate to FCPA.