The US tax authority said Thursday it had asked for an independent investigation into rare, intrusive audits of two ex-FBI heads who were prominent adversaries of former president Donald Trump.

James Comey, the FBI director until he was sacked by Trump in 2017, and Andrew McCabe, Comey's deputy and temporary replacement, were both subjected to the Internal Revenue Service reviews while the Republican billionaire was in office.

Individuals are supposed to be picked at random for the IRS's National Research Program audits, making the chances of Comey being singled out in 2017 about one in 30,000, while McCabe's odds in 2019 were about one in 20,000.

The revelation, first reported by The New York Times, raised questions over how two men who ran the nation's premier domestic police agency and were seen by Trump as among his most high-profile foes could both have been selected.