Washington • President Donald Trump on Wednesday named Robert O’Brien, his chief hostage negotiator and an established figure in Republican policy circles, as his new national security adviser.

O’Brien, the fourth person in two years to hold the job, becomes the administration’s point person on national security amid rising tensions with Iran following the weekend attack on Saudi oil installations and fresh uncertainty in Afghanistan after the halt of peace talks with the Taliban.

The announcement of O’Brien’s selection comes a week after Trump ousted John Bolton from the post, citing policy disagreements. O’Brien, who made headlines in July when he was dispatched to Sweden to monitor the assault trial of American rapper A$AP Rocky, was among five candidates Trump said Tuesday were under consideration.

"He's worked with me for quite awhile now on hostages and we have a tremendous track record on hostages," Trump said Wednesday on a tarmac in Los Angeles, hours after revealing the pick on Twitter. "Robert has been fantastic. We know each other well.

O'Brien, standing alongside Trump, said it was a "privilege" to be picked.

"We've got a number of challenges," he said, adding that the administration's focus will continue to be on keeping the U.S. safe and rebuilding the military. He said he would advise Trump privately on the situation in Saudi Arabia.

O'Brien is a member of The Church of Jesus Christ of Latter-day Saints and will be the highest ranking member of the faith in the administration.

Sen. Mike Lee, a Utah Republican and also a fellow Latter-day Saint, praised O'Brien.

“I’ve had the opportunity to work with Mr. O’Brien during his time in the State Department on issues regarding religious liberty and Utahns abroad in Russia, China and throughout the world,” Lee said in a statement. “He is an incredible champion of religious freedom around the globe and I am confident that he will excel in his role as national security adviser to the president.”

O'Brien also served as a top foreign policy adviser to Mitt Romney during his 2012 presidential bid.

Romney, who is also a Latter-day Saint and now a Utah senator, offered his congratulations on Twitter.

“As the special presidential envoy for hostage affairs, [O’Brien] has doggedly pursued the release of American hostages abroad,” Romney tweeted. “He is a man of the highest integrity.”

Trump abruptly forced out Bolton on Sept. 10, after he and his hawkish national security adviser found themselves in disagreement over the administration’s approach to Iran, Afghanistan and a host of other global challenges. The sudden exit marked the latest departure of a prominent voice of dissent from Trump’s inner circle as the president has grown more comfortable following his gut instinct over the studious guidance offered by his advisers.

As the special presidential envoy for hostage affairs at the State Department, O'Brien worked closely with the families of American hostages and advised administration officials on hostage issues. He helped secure the release in February of American citizen Danny Burch, who was freed after 18 months in captivity in Yemen.

He has also worked on the case of missing U.S. journalist Austin Tice, who was captured in Syria in 2012. O'Brien has said he is confident Tice is still alive.

The White House sent O'Brien to Sweden to monitor the case of A$AP Rocky, who was charged with assault. The rapper, whose real name is Rakim Mayers, was permitted to return to Los Angeles to await the verdict of a Swedish court, which found him guilty in a street brawl.

Last month, Hua Qu, the wife of a Princeton University graduate student detained in Iran, told reporters that she would like to see the same level of personal attention from the government as A$AP Rocky received.

On Wednesday, a Twitter account that advocates for the release of the student, Xiyue Wang, congratulated O'Brien on the appointment but also said: "We watched with frustration as this administration put all its energies into releasing celebrity A$AP Rocky from Sweden; meanwhile, (asterisk)multiple(asterisk) Americans suffer under terrible conditions in #Iran. Let's put all Americans first, not just famous ones. #AmericansFirst."

O'Brien previously helped lead the department's public-private partnership for justice reform in Afghanistan during the Bush and Obama administrations.

He began to emerge as a front-runner to replace Bolton last week when it became clear that an early favorite, Iran envoy Brian Hook, would face opposition from hawks who think he has not been tough enough on Iran, according to Republicans familiar with the matter.

Another short-listed candidate, the North Korea envoy, Stephen Biegun, was taken out of the mix when Secretary of State Mike Pompeo suggested he might be better placed as deputy secretary of state to replace John Sullivan, who is widely expected to be nominated to be the next U.S. ambassador to Russia, officials said.

That spot is soon to be vacant with the departure of Jon Huntsman, who has served roughly two years in the diplomatic post. The former Utah governor is returning to the Beehive State and reportedly considering running again for his previous post.

From 2008 through 2011, O'Brien was a presidentially appointed member of a government committee that advises on issues related to the trafficking of antiquities and other cultural items. In 2005, President George W. Bush nominated O'Brien to be U.S. representative to the U.N. General Assembly, where he worked with Bolton. O'Brien was confirmed by the Senate.

He also was an adviser on the Republican presidential campaigns of former Gov. Scott Walker of Wisconsin and Sen. Ted Cruz of Texas.

Earlier in his career, O'Brien was a senior legal officer for the U.N. Security Council commission that decided claims against Iraq that arose from the Gulf War. He was a major in the U.S. Army Reserve.

O'Brien has a law degree from the University of California, Berkeley, and co-founded a law firm in Los Angeles focused on litigation and international arbitration issues. O'Brien is the author of "While America Slept," a collection of essays on U.S. national security and foreign policy billed as a "wake-up call to the American people."

The book warned that the world had become more dangerous "under President Obama's lead-from-behind foreign policy."

Associated Press writer Matthew Lee and The Salt Lake Tribune’s Thomas Burr contributed to this report.

Jon Huntsman is a brother of Paul Huntsman, owner and publisher of The Salt Lake Tribune.