Robert O'Brien has until now served as Trump's envoy for situations involving US hostages abroad.

US President Donald Trump on Wednesday named his pointman for hostage situations, Robert O'Brien, to replace his hawkish national security advisor sacked last week.

"I have worked long & hard with Robert. He will do a great job!" Donald Trump tweeted

Last week, Donald Trump abruptly fired John Bolton, a vigorous proponent of using US military force abroad and one of the main hawks in the administration on Iran.

Robert O'Brien has until now served as Donald Trump's envoy for situations involving US hostages abroad.

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

Bolton, by contrast, was a highly controversial figure in Washington. His instincts for an aggressive, interventionist foreign policy were at odds with Donald Trump's more isolationist stance.

Donald 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."

Robert O'Brien will become the fourth national security advisor in Donald Trump's tumultuous first term.