WASHINGTON -- Robert Mueller's office has referred to the FBI allegations that women were "offered money to make false claims" about the special counsel, according to Mueller's spokesman.

In a statement, spokesman Peter Carr says that once the office learned of the allegations, it immediately referred the matter to the FBI for investigation.

The statement didn't specify what the claims were, but the referral to the FBI -- and a rare public statement about it from the special counsel's office -- suggests that Mueller's office believed there was a potential crime for federal law enforcement to investigate.

The attempt to spread what Mueller's office says are false claims about him also appears to be an effort to discredit the former FBI director as his team enters a critical stage of its investigation into whether the Trump campaign co-ordinated with Russia and whether the president attempted to obstruct justice.

Mueller's office has charged 32 people so far, with four former associates of the president pleading guilty and agreeing to co-operate with investigators. A grand jury in recent weeks has heard testimony centred on another former Trump aide, Roger Stone.