Former US ambassador to the UN John Bolton will replace HR McMaster as US National Security Advisor next month, President Donald Trump said Thursday.

"I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor," Trump wrote on Twitter.

McMaster had done "an outstanding job and will always remain my friend," Trump added.

McMaster's decision to depart was a mutual decision, according to a White House official.

"I am thankful to President Donald J. Trump for the opportunity to serve him and our nation as national security advisor," McMaster said in a statement.

McMaster, a 1984 graduate of the US Military Academy at West Point, has served in conflicts in Afghanistan, Iraq and the 1991 Gulf War during a 34-year military career.

Trump had asked McMaster to stay on until early April to ensure a smooth transition and McMaster agreed, the official said.

Bolton, who will become Trump's third national security advisor, served as UN ambassador under George W Bush from 2005 to 2006, and strongly defended Bush's policies including the war in Iraq.

The 69-year-old was a highly controversial appointment to that role and was liked by some diplomats during his term, but irritated many by his strong-armed approach to diplomacy.

Last week, Trump fired Secretary of State Rex Tillerson, replacing him with CIA director Mike Pompeo, in a move seemingly part of an effort to reshape his foreign policy team ahead of a potential meeting with North Korean leader Kim Jong Un.