Defense Secretary James Mattis 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 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.

"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 replacing Rex Tillerson 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.