After one year in his job, national security adviser H.R. McMaster is expected to depart his White House position soon, U.S. officials tell NPR's Pentagon correspondent Tom Bowman. Despite a denial from the Trump administration, the only thing that is reportedly holding up McMaster's departure is a transition plan.McMaster's exit has been the subject of rumors, in a similar way that outgoing Secretary of State Rex Tillerson had been viewed as not long for life under President Trump. Officials say McMaster, 55, could leave within weeks, possibly within a month or so."We don't get a sense of when this will happen," Tom says, "but we're told it will likely happen."Information about Trump's desire to replace McMaster has been solidified by sources, despite a denial from White House press secretary Sarah Huckabee Sanders, who tweeted Thursday night that she had just spoken to Trump and McMaster and that "contrary to reports they have a good working relationship and there are no changes at the NSC [National Security Council].""Trump has publicly said that he respects H.R. McMaster, but their relationship has not been an easy one," Tom tells NPR's Rachel Martin on Morning Edition. "There were some policy differences: H.R. favored an enduring commitment in Afghanistan, he served there for a time and actually wanted more troops sent over there. Others in the White House disagreed with that."As for any possible personal rift between Trump and McMaster, Tom says, "People in the Army say he's 'all transmit,' as they would put it: He likes to lecture. That rubs some people the wrong way — and the sense is, that rubs Trump the wrong way."Some of McMaster's friends had reportedly advised him not to work for the new president — Tom cites some of their warnings: "The president is mercurial, he has no background in military policy, foreign policy — so it was a tough job for him to take. But ... [McMaster] is very aggressive, has a big ego. I'm sure he thought that he could do a pretty good job here. But clearly, he's on his way out."Trump publicly chastised McMaster last month, after the national security adviser said that "the evidence is now really incontrovertible" that Russia had interfered in the 2016 election that put Trump in the Oval Office.That statement spurred Trump to tweet, "General McMaster forgot to say that the results of the 2016 election were not impacted or changed by the Russians" and to deny any collusion.When McMaster accepted the national security adviser job in the Trump administration one year ago, he was stepping into a post that had been vacated by the firing of Michael Flynn, who is also a figure in the Russian investigation. In early 2017, reports cited McMaster's reluctance to take the position — and when he finally did take the job, McMaster retained his active-duty status as a lieutenant general in the Army.Tom says that he was told that while McMaster's departure is definite, the timing is uncertain — possibly within a month or two, officials said. But as Tom adds, "With this White House, with the president tweeting, it could happen today. We have really no sense when it could happen."When Trump fired Tillerson earlier this week, the news came in a tweet — evidently surprising Tillerson, despite long-swirling rumors and denials about his job security. Copyright 2018 NPR. To see more, visit http://www.npr.org/.