U.S. President Donald Trump on Wednesday named his envoy for hostage negotiations, Robert O'Brien, as the administration's new national security adviser.

Trump made the announcement on Twitter, saying that he is pleased to announce that he will name O'Brien, currently serving as the “very successful” special presidential envoy for hostage affairs at the State Department, as new national security adviser. 

Trump added he has worked long and hard with the new pick and that he will do a great job. 

The announcement came a little over a week after previous national security adviser John Bolton was sacked over disagreements on foreign policies on countries including North Korea.

O'Brien has served as Trump's envoy for situations involving U.S. hostages abroad.

The new national security adviser, Trump’s fourth in two years, is known to have the strong backing of U.S. Secretary of State Mike Pompeo and could further boost Pompeo's voice in the administration's foreign policy decisions.