Preet Bharara, the U.S. attorney for Manhattan, said he had "opened a criminal investigation regarding matters to which the Panama Papers are relevant," The Guardian wrote.

Bharara has written to the International Consortium of Investigative Journalists (ICIJ), which coordinated the unprecedented leak of 11.5m files from offshore law firm Mossack Fonseca, to ask for further information to assist with his criminal investigation.

Read alsoPanama govt announces creation of 'panel of experts'The U.S. attorney general's office was unable to provide any further details about the criminal investigation because it is ongoing.

Bharara, who as U.S. attorney general for the southern district of New York has led several crusades against criminal wrongdoing in the financial sector, is already investigating several of the more than 200 U.S. citizens named in the papers.

U.S. Department of Justice