"H.R. McMaster has served his country with distinction for more than 30 years. He has won many battles and his bravery and toughness are legendary,"Trump said in a statement.

"General McMaster's leadership of the National Security Council staff has helped my administration accomplish great things to bolster America's national security."

McMaster had struggled for months to impose order not only on a fractious national security team but on a president who resisted the sort of discipline customary in the military.

Although McMaster has been a maverick voice at times during a long military career,the Washington foreign policy establishment had hoped he would keep the president from making rash decisions.

Yet the president and the general,who had never met before Trump interviewed McMaster for the post,had little chemistry from the start,and often clashed behind the scenes.

McMaster's serious,sombre style and preference for order made him an uncomfortable fit with a president whose style is looser,and who has little patience for the detail and nuance of complex national security issues. They had differed on policy,with McMaster cautioning against ripping up the nuclear deal with Iran without a strategy for what would come next,and tangling with Trump over the strategy for US forces in Afghanistan.

Their tensions seeped into public view in February,when McMaster said at a security conference in Munich that the evidence of Russian interference in the 2016 presidential election was beyond dispute. The statement drew a swift rebuke from the president,who vented his anger on Twitter.

"General McMaster forgot to say that the results of the 2016 election were not impacted or changed by the Russians and that the only Collusion was between Russia and Crooked H,the DNC and the Dems,"Trump wrote,using his campaign nickname for Hillary Clinton."Remember the Dirty Dossier,Uranium,Speeches,Emails and the Podesta Company!"

Trump selected McMaster last February after pushing out Michael Flynn,his first national security adviser,for not being forthright about a conversation with Russia's ambassador at the time. (Flynn has since pleaded guilty of making a false statement to the FBI and is co-operating with Robert Mueller,the special counsel investigating Russia's interference in the 2016 election.)

McMaster carried out a slow-rolling purge of hard-liners at the National Security Council who had been installed by Flynn and were allied ideologically with Stephen Bannon,Trump's former chief strategist,earning the ire of conservatives who complained that his moves represented the foreign policy establishment reasserting itself over a president who had promised a different approach.

McMaster's position at the White House had been seen as precarious for months,and he had become the target of a concerted campaign by hard-line activists outside the administration who accused him of undermining the president's agenda and pushed for his removal,even creating a social media effort branded with a #FireMcMaster hashtag.

Last summer,Trump baulked at a plan McMaster presented to bolster the presence of US forces in Afghanistan,although the president ultimately embraced a strategy that would require thousands more US troops.

McMaster had been among the most hard-line administration officials in his approach to North Korea,publicly raising the spectre of a"preventive war"against the North. He was among those who expressed concerns about Trump's abrupt decision this month to meet Kim Jong Un,according to a senior official.

New York Times,Bloomberg