U.S. President Donald Trump announced on Wednesday that he will name Robert O'Brien, a senior hostage affairs official at the State Department, as his next national security advisor.

"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 Advisor," Trump said in a morning tweet.

O'Brien will replace John Bolton, the third U.S. national security advisor during the Trump administration, who ended his tenure over a week ago, as his relationship with Trump frayed over policy differences.

"I have worked long & hard with Robert. He will do a great job!" Trump added in the tweet.

O'Brien, a long-time lawyer before taking various government posts in the past administrations, was among the five finalists Trump revealed on Tuesday.

He takes the job as the Trump administration is facing a number of foreign-policy challenges, including its increasingly tense relations with Iran.

The position of national security advisor does not require Senate confirmation.