The Pentagon's inspector general has opened an investigation of Navy Rear Adm. Ronny Jackson, the White House doctor whose nomination to head the Department of Veterans Affairs was derailed by allegations of misconduct, officials said Monday.

A spokesman for the IG's office, Bruce Anderson, announced the probe but said he could not comment on the specific allegations.

Jackson has denied the allegations, which surfaced during the brief period his nomination was under consideration by the Senate. After being nominated, he stepped aside from leading President Donald Trump's medical care, but he remains a member of the White House medical unit. He has been with the White House unit since 2006.

Jackson withdrew his nomination in April after allegations by current and former colleagues raised questions about his prescribing practices and leadership ability, including unsubstantiated accusations of drunkenness on the job.

The allegations were referred to the Pentagon's inspector general for evaluation. After an initial assessment and review, the IG's office decided a formal investigation was warranted, Anderson said.

In March, Trump nominated Jackson for promotion to the rank of two-star admiral. Some members of the Senate Armed Services Committee, which would vote on a promotion, had called for a Pentagon inspector general investigation.

Trump has called Jackson a "great man (who) got treated very, very unfairly" after his VA nomination.