Citing two unnamed sources, Bloomberg said Rosenstein brought up the matter and offered the reassurance in a meeting with Trump at the White House on April 12, and that the development "helped tamp down the president’s desire to remove Rosenstein or Mueller."

Trump told some of his closest advisers after the meeting that it is not the right time to remove either man since he is not a target of the probe, Bloomberg reported.

The Justice Department told Reuters it does not comment on conversations with the president. The White House did not immediately respond to a request for comment.

Mueller is investigating allegations of Russian interference in the 2016 U.S. election and possible collusion with the Trump campaign.

Russia has denied meddling in the election. Trump has said there was no collusion and has repeatedly called the Mueller probe a "witch hunt," raising concerns he might try to fire the special counsel or Rosenstein, who oversees the investigation.

Under Justice Department policy, a target is someone who is believed to have committed a crime and is likely to face charges, while a subject is someone whose conduct is within the scope of an investigation, said Lisa Kern Griffin, a former federal prosecutor and a professor at Duke University School of Law.

The Washington Post reported earlier this month that Trump is a subject of the probe.

Griffin said the assurance from Rosenstein is not significant because the president could yet become a target of the investigation.

"It is possible to progress from being a subject to being a target if the necessary substantive and structural support emerges later," Griffin said.

Griffin said one reason Trump is being treated as a subject, rather than a target, may be that Rosenstein is operating under assumption that a sitting president cannot be indicted.