US President Donald Trump on Wednesday announced the nomination of Robert O’Brien as national security adviser to succeed John Bolton.

“I am delighted to announce the nomination of Robert O’Brien as our new national security adviser. I have worked with him for a long time. He will do a great job,” Trump said on his official Twitter account.

O’Brien is a law firm and is currently the Presidential Special Envoy for Hostage Affairs at the State Department.

Trump surprisingly announced the dismissal of Bolton, a leading hawk in the US administration, amid differences over dealing with foreign policy challenges such as North Korea, Iran, Afghanistan and Russia.

Trump described Bolton as a “cruel person” and said he was “inclined to military interventions, while the invasion of Iraq, for example, was a grave mistake,” as he put it.

“I told John Bolton … that he no longer needed his services at the White House,” he said.

Bolton, 70, who took over the job in April 2018 from HR McMaster, was at odds with Secretary of State Mike Pompeo, known for his loyalty to Trump.