Former Secretary of State Rex Tillerson told the Washington Post Monday he'd never tried to subvert President Trump, after the outlet reported ex-U.S. Ambassador to the UN Nikki Haley makes the claims in a new book.

During my service to our country as the Secretary of State, at no time did I, nor to my direct knowledge did anyone else serving along with me, take any actions to undermine the President."

— Tillerson statement to WashPost

Driving the news: Per WashPost, Trump loyalist Haley writes in her memoir, "'With All Due Respect," that Tillerson and former Chief of Staff John Kelly tried and failed to recruit her to subvert Trump in order to "save the country."

(Kelly told WashPost if giving Trump "the best and most open, legal and ethical staffing advice" from across government so the president "could make an informed decision is 'working against Trump,' then guilty as charged.")

The big picture: Haley left on good terms with the president. She's popular within the Republican Party and is seen by many as a potential successor to Trump.

In contrast, Trump fired Tillerson — who had broken with him on several key international issues and reportedly called Trump a "f****ing moron." Trump tweeted in May that Tillerson was as "dumb as a rock" after reports that Tillerson disparaged his preparation for a 2017 meeting with Russian President Vladimir Putin.

What he's saying: Tillerson said in his statement that his conversations with Trump "in the privacy of the Oval Office were always candid, frank, and my recommendations straightforward." "Once the President made a decision, we at the State Department undertook our best efforts to implement that decision," Tillerson said.

"Ambassador Haley was rarely a participant in my many meetings and is not in a position to know what I may or may not have said to the President. I continue to be proud of my service as our country’s 69th Secretary of State."

Go deeper: Tillerson's farewell: "This can be a very mean-spirited town"