Washington, Sep 18 (IANS): US President Donald Trump on Wednesday named Robert O'Brien, a State Department official who has specialized in hostage issues, as his new National Security Adviser (NSA).

"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 tweeted.

"I have worked long and hard with Robert. He will do a great job!" O'Brien will be the fourth National Security Adviser. He will replace John Bolton, whom Trump fired last week after a string of disagreements.

O'Brien had been on Trump's list of finalists and the President had praised him as "fantastic". He was integrally involved in the release of rapper ASAP Rocky by Swedish authorities in August.

The rapper had been detained and charged with assault in connection with a June 30 scuffle between his entourage and a 19-year-old man in Stockholm.

The NSA role does not require confirmation by Congress.

O'Brien had served as a major in the US Army Reserve. He was also appointed in 2005 to serve as a representative to the UN General Assembly. While at the UN, he worked with Bolton, who was then the Ambassador.