Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE has been shut out of crucial Trump administration decisions, according to an NBC News report published Monday.

The network said, for example, that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE did not discuss pausing United States military drills with South Korea with Mattis before the landmark summit in Singapore with North Korean leader Kim Jong Un.

One senior White House aide told NBC News that Mattis was aware the president could choose to halt the exercises.

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"He knew that it could have been offered. The president was in the room with the leader of North Korea and made the decision,” that official said.

Trump has grown fed up with Mattis for stalling his policy choices, the report said.

While two former Trump administration officials told NBC that Trump faulted Mattis for the way in which changes to the military's policy on transgender troops unfolded, another source told the news outlet that the policy change was a lengthy push.

The report also referred to the recent changes in Trump's Cabinet, including Secretary of State Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE replacing Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE and national security adviser John Bolton replacing H.R. McMaster.

Mattis stopped openly opposing Trump's choice to withdraw from the Iranian nuclear accord after McMaster and Tillerson departed, according to the report.