President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE has ordered a significant staff cut at the White House National Security Council (NSC), Bloomberg reported late Friday, citing five people familiar with the matter.

Some of the individuals reportedly said the move was part of an effort under new national security adviser Robert O’Brien, who told officials of the reduction alongside acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE this week.

Two people familiar with the matter told Bloomberg that the change was due to O'Brien's leadership and Trump's wish to increase agency efficiency. The news outlet noted the NSC grew under former President Obama and that about 310 people work there now.

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However, Bloomberg noted that the changes also come as Trump faces scrutiny over a whistleblower complaint regarding his July phone call with Ukrainian President Volodymyr Zelensky, which has sparked an impeachment inquiry in the House.

The whistleblower said that a transcript of Trump's call with the Ukrainian leader was stored in a computer system managed by the NSC. The White House later reportedly confirmed that a partial transcript of the call was moved to the highly classified server at the direction of White House lawyers.

The New York Times reported that the whistleblower was a CIA officer who had been assigned to work at the White House at some point. Trump on Friday night retweeted a claim that the person was a CIA official who was at the NSC under Obama, although evidence was not provided to support this allegation.

Trump named O'Brien as his new national security adviser last month. He replaced John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, with whom Trump had a sometimes contentious relationship.

The whistleblower complaint and impeachment inquiry focus primarily on the Trump-Zelensky call in which the U.S. president asked the Ukrainian leader to look into Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

The Hill has reached out to the White House for comment.