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 in 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.

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

Trump abruptly forced out Bolton on Sept. 10, after he and his hawkish national security adviser found themselves in strong 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 and Marine Corps veteran Austin Tice, who was captured in Syria in 2012. O’Brien has said he is confident Tice is still alive.

In this March 6, 2019, file photo, former U.S. hostage in Yemen, Danny Burch, left, listens as President Donald Trump speaks, Wednesday, March 6, 2019, in the Oval Office of the White House in Washington, next to Vice President Mike Pence, and Special Presidential Envoy for Hostage Affairs Robert O'Brien. At back, to the right of Trump, is then-national security adviser John Bolton. (Jacquelyn Martin/AP)
In this March 6, 2019, file photo, former U.S. hostage in Yemen, Danny Burch, left, listens as President Donald Trump speaks, Wednesday, March 6, 2019, in the Oval Office of the White House in Washington, next to Vice President Mike Pence, and Special Presidential Envoy for Hostage Affairs Robert O'Brien. At back, to the right of Trump, is then-national security adviser John Bolton. (Jacquelyn Martin/AP)

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 that 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, North Korea envoy, Stephen Biegun, was taken out of the mix when 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.

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, former Gov. Mitt Romney of Massachusetts 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 contributed to this report.