(CNN) White House chief of staff John Kelly grew so frustrated on the day Veterans Affairs Secretary David Shulkin was fired that both Homeland Security Secretary Kirstjen Nielsen and Defense Secretary James Mattis attempted to calm him down, The Washington Post reported Saturday, in a story based on interviews with more than a dozen administration officials and confidants and advisers to the President.

CNN has previously reported on tensions between the President and his chief of staff, including the fact that Kelly has threatened to quit in the midst of clashes with Trump, according to people familiar with the incidents. But Kelly has told allies that he believes it is his duty to help implement the President's agenda.

Trump has started to bypass Kelly, who has sought to control access to the President, in a variety of ways, CNN has reported , such as in conversations with foreign leaders, policy decisions, and in executing recent staffing changes, according to individuals with knowledge of the matter. Kelly, meanwhile, has sought to bring further order to the policymaking process, including by naming Chris Liddell as his deputy for policy implementation.

Last month, Kelly even attempted to dispel rumors that he was on the outs with the President, telling aides "I'm in," while Trump told advisers that his chief of staff was "100% safe."

The Post reports that when Trump announced via Twitter last month that Shulkin would be replaced by his personal physician, Ronny Jackson, Kelly became so exasperated that Nielsen, his former deputy at the White House and chief of staff when he ran the Department of Homeland Security, and Mattis sought to calm him down, according to three individuals with knowledge of what happened. It's unclear why Kelly was so frustrated, but the Post reports that he said, "I'm out of here, guys."

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