US President Donald Trump today fired National Security Adviser Lt Gen H R McMaster, replacing him with former UN Ambassador John Bolton, the latest high-level shakeup in the White House. Reports of McMaster's ouster had surfaced last week but the White House had dispelled them saying there were no changes at the National Security Council (NSC). Trump, however, took to Twitter today to make an announcement in this regard. "I am pleased to announce that, effective 4\/9\/18, John Bolton will be my new National Security Adviser," Trump said on Twitter. "I am very thankful for the service of General H R McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4\/9," Trump said. McMaster was Trump's second national security adviser after Michael Flynn was dismissed last year for misleading Vice President Mike Pence about his contacts with the Russian ambassador to the US. The NSC is one of the most powerful wing of the White House that drives the administration's national security agenda. The announcement of McMaster's ouster, who the White House said had a "good working relationship" with Trump, is the latest in a string of high-profile White House departures since President Trump took office in January 2017. The announcement came just days after he replaced secretary of state Rex Tillerson with his close ally, CIA Director Mike Pompeo. That move, Trump said, was in preparation for upcoming top-level nuclear talks with North Korean leader Kim Jong Un. According to a White House official, Trump and McMaster mutually agreed that the latter will resign as the National Security Adviser (NSA). He will retire from the US Army after 34 years of distinguished service. "The two have been discussing this for some time. The timeline was expedited as they both felt it was important to have the new team in place, instead of constant speculation. This was not related to any one moment or incident, rather it was the result of ongoing conversations between the two," the official said. Trump and McMaster both released written statements thanking each other, with the president describing McMaster as an architect of his "America First" foreign policy. "He has won many battles and his bravery and toughness are legendary. General McMaster's leadership of the National Security Council staff has helped my administration accomplish great things to bolster America's national security," Trump said. "He helped develop our America First National Security Strategy, revitalise our alliances in the Middle East, smash ISIS, bring North Korea to the table, and strengthen our nation's prosperity. This work and those achievements will ensure that America builds on its economic and military advantages. I thank General McMaster and his family for their service and wish them the very best," Trump said. McMaster said after 34 years of service, he is requesting retirement from the US Army effective this summer after which he will leave public service. "Throughout my career, it has been my greatest privilege to serve alongside extraordinary service members and dedicated civilians. "I am thankful to President Donald J Trump for the opportunity to serve him and our nation as national security adviser. I am grateful for the friendship and support of the members of the National Security Council who worked together to provide the President with the best options to protect and advance our national interests," McMaster said. The White House has described Bolton, who replaces McMaster, as one of the strongest voices and experts on the full range of national security issues and challenges facing the country. Bolton served as US Permanent Representative to the United Nations from 2005-2006 and as Under Secretary of State for Arms Control and International Security from 2001-2005. Bolton, during his tenure at the United Nations, was a leading voice on the need for the Security Council to take strong and meaningful action against international proliferation and terrorism. Along with France's ambassador, Bolton led the Security Council to approve a unanimous resolution to end the summer 2006 Hezbollah war on Israel, to authorise UN peacekeepers and to create an arms embargo against Hezbollah. He also assembled an international coalition that blocked the bid of Hugo Chavez, Venezuela's Marxist strongman, to join the Security Council. As Under Secretary, he repeatedly advocated tough measures against the nuclear weapons programs of both Iran and North Korea, and the proliferation of weapons of mass destruction generally. He led negotiations for America to withdraw from the 1972 Anti-Ballistic Missile Treaty so that the Bush administration could proceed with a national missile-defence programme.