US President Donald Trump is seeking to replace Defence Secretary Jim Mattis mainly because of the sharp differences between them, a report in The Washington Post has claimed. The report quoted an unnamed White House official as saying that speculation about a replacement was now "more real than ever" after revelations in famed investigative journalist Bob Woodward's upcoming book titled "Fear: Trump in the White House". However, Trump, soon after publication of the report denied such a move and said Mattis is doing a fantastic job. Mattis is in India along with Secretary of State Mike Pompeo to attend the maiden India-US 2+2 dialogue being hosted by their Indian counterparts External Affairs Minister Sushma Swaraj and Defence Minister Nirmala Sitharaman. "Well before this week's revelations about President Trump's interactions with Defense Secretary Jim Mattis in Bob Woodward's new book, officials inside the White House have been actively discussing who will replace Mattis at the Pentagon \u2014 whenever he might step down,\u201d the daily reported. In the forthcoming book being released next week, Woodward claims that Mattis told associates that Trump "acted like \u2014 and had the understanding of \u2014 a fifth- or sixth-grader". The 448-page book is scheduled to hit stores on September 11 and claims to give an insider's account on the White House working and decision-making process in Trump's presidency. Mattis would leave his post sometime over the next few months, according to the daily. "The speculation about who replaces Mattis is now more real than ever," a senior White House official was quoted as saying in the report. "The president has always respected him. But now he has every reason to wonder what Mattis is saying behind his back. The relationship has nowhere to go but down, fast," the official said. Mattis a day earlier had issued a statement denying his remarks in Woodward's book. "The contemptuous words about the president attributed to me in Woodward's book were never uttered by me or in my presence. "While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility," the defence secretary had said. Denying any move to replace the defence secretary, Trump described Mattis as a terrific person and said "he is doing a fantastic job as secretary." "He will stay. We're very happy with him, we're having a lot of victories, we are having victories that people don't even know about, and he is highly respected all over the world," the president said. "I think because the book is a total piece of fiction. He (Mattis) was totally, not even misquoted, he never made the statement. "I have heard over the years that General Mattis is an intellect, and when I read his statement, I asked him whether or not this was true," he told reporters at an White House event.