U.S. President Donald Trump says he has chosen Robert O'Brien to replace John Bolton as White House national-security adviser.
"I have worked long & hard with Robert. He will do a great job!" Trump said in a tweet on September 18 in announcing the appointment.
The 53-year-old O'Brien is currently a presidential envoy for hostage affairs at the U.S. Department of State.
The position does not require Senate confirmation.
Trump has been searching for a new national-security adviser since he fired Bolton on September 10, saying they "disagreed strongly" on many issues.
Bolton, who was Trump's third national-security adviser since he was inaugurated in January 2017, has said that he offered to resign the evening before the president fired him.
In his current position, O'Brien has worked closely with the families of U.S. hostages and advised administration officials on hostage-related matters. In February, he helped attain the release Danny Burch, a U.S. citizen who was freed after 18 months in captivity in Yemen.
He previously helped lead the State Department's public-private partnership for justice reform in Afghanistan during the administrations of Presidents George W. Bush and Barack Obama.
O’Brien, in his 2016 book titled While America Slept, criticized what he described as Obama's attempt to present a more collaborative, dovish United States.