LOS ANGELES — President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Wednesday announced he intends to name the administration's envoy for hostage negotiations, Robert O'Brien, as his next national security adviser.

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

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. I have worked long & hard with Robert. He will do a great job! — Donald J. Trump (@realDonaldTrump) September 18, 2019

O'Brien will replace John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, whom Trump fired last week and later criticized for failing to align with the administration's agenda on North Korea and Venezuela.

The role of national security adviser does not require Senate confirmation.

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O'Brien will be Trump's fourth national security adviser in less than three years. He follows Michael Flynn, H.R. McMaster and Bolton.

He steps into a challenging role as Trump faces crossroads on how to handle escalating tensions in Iran, stalled negotiations on denuclearization in North Korea, a worsening situation in Venezuela and other global issues.

O'Brien must also navigate advising Trump without overstepping, something the more hawkish Bolton was apparently unable to do.

Trump appeared alongside O’Brien in California before boarding Air Force One for a fundraiser in San Diego Wednesday afternoon, praising him as “fantastic” and touting his administration’s track record on bringing American hostages home.

“He’s worked with me for quite a while now on hostages and we’ve had a tremendous track record,” Trump told reporters. “We brought a lot of people home and we haven’t spent any money.”

O’Brien said he looked forward to his new role, saying the administration has notched a number of foreign policy wins while acknowledging that challenges lie ahead.

“It’s a privilege to serve with the president and I look forward to another year and a half of peace through strength. We’ve had tremendous foreign policy successes under President Trump’s leadership. I expect those to continue,” O’Brien said.

“We have a number of challenges, but there’s a great team in place with Secretary Pompeo and Secretary Esper and Secretary Mnuchin and others,” O’Brien continued. “I look forward to working with them and working with the president to keep America safe and continue to rebuild our military and really get us back to a peace through strength posture.”

Tapped in 2018 as the chief hostage negotiator for the State Department, O'Brien has worked to secure the release of Americans abroad. He was present for proceedings in Sweden involving rapper A$AP Rocky, whom Trump had urged the Swedish government to free after his arrest on assault charges.

O'Brien has worked in diplomatic relations for years. Former President George W. Bush chose him to serve as a representative to the United Nations General Assembly in 2005, and during the Bush and Obama administrations he served as co-chairman of a State Department initiative to train judges, prosecutors and defense lawyers in Afghanistan in an effort to bolster the justice system there.

The president on Tuesday shared a shortlist of candidates under consideration for the job with reporters aboard Air Force One while en route to California for fundraisers.

O'Brien was among those listed, along with former deputy national security adviser Ricky Waddell, Energy Department official Lisa Gordon-Hagerty, former CIA analyst Fred Fleitz and Army Gen. Keith Kellogg.

Updated at 3:12 p.m.