U.S. President Donald Trump said on Twitter that he will name Robert C. O'Brien, a senior U.S. hostage negotiator as the country's new national security adviser, making him the fourth person to hold the post in the Trump administration.

"I am pleased to announce that I will name Robert C. O'Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Adviser," Trump said in a morning tweet. "I have worked long & hard with Robert. He will do a great job!"

O'Brien will replace John Bolton, who ended his tenure over a week ago, as his relationship with Trump frayed over policy differences. 

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O'Brien, who has served as Trump's envoy for situations involving U.S. hostages abroad since May 2018, has a long history in Republican foreign policy circles. 

He is an attorney from Los Angeles who has served as a foreign policy adviser to several Republican presidential campaigns, handled a number of high profile legal cases and previously served in several State Department positions, including as an alternative representative to the U.N. General Assembly in 2005.

He comes into the new job with backing from Secretary of State Mike Pompeo and senior Republicans in Congress. 

Bolton, by contrast, a vigorous proponent of using U.S. military force abroad and one of the main hawks in the administration on Iran, was a highly controversial figure in Washington. 

Trump said that one reason he fired Bolton was because he "wasn't getting along with people in the administration who I consider very important" and "wasn't in line with what we were doing." 

Trump had been weighing a number of potential replacements after Bolton left the post last week following disagreements with the president over his handling of North Korea and Venezuela.

His previous national security advisers were H.R. McMaster, who was replaced by Bolton in March 2018, and Michael Flynn, who was fired shortly after taking the role and later pleaded guilty to lying to the FBI.

(With input from agencies)