File photo taken on May 16, 2017 shows H.R. McMaster answers questions during a press briefing at the White House in Washington D.C., the United States. U.S. President Donald Trump tweeted on March 22, 2018 that John Bolton, former U.S. Ambassador to the United Nations, will replace General H.R. McMaster to be his new national security adviser. (Xinhua/Yin Bogu)

WASHINGTON, March 22 (Xinhua) -- U.S. President Donald Trump tweeted on Thursday that John Bolton, former U.S. ambassador to the United Nations, will replace General H.R. McMaster to be his new national security adviser.

Trump added that the announcement will be effective on April 9, when there will be an official contact handover.

"I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend," Trump added.

Later, U.S. Ambassador to the United Nations Nikki Haley also tweeted that McMaster's "selfless courage and leadership has inspired all of us. Most of all, thank you for your friendship."