Rice Named New U.S. National Security Adviser

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WATCH: Obama Makes Rice His National Security Chief

WASHINGTON -- U.S. President Barack Obama has named Susan Rice, Washington's ambassador to the United Nations, to be his top adviser on national security. She will replace Tom Donilon, who is resigning early next month, as the president's primary consultant on foreign policy matters.

"Today I am wistful to announce that after more than four years of extraordinary service Tom [Donilon] has decided to step aside at the beginning of July," Obama announced from the White House's Rose Garden. "And I am extraordinarily proud to announce my new national security adviser, our outstanding ambassador to the United Nations, Susan Rice."

Obama described Rice, a close ally of the president, as "fearless," and "an exemplary public servant."

The 48-year-old previously served on the National Security Council during the Clinton administration. At the UN, she helped push through tough sanctions on Iran and North Korea, defended Israel, and advocated for human rights.

According to "The New York Times," Rice has also maintained "an influential role in internal debates over questions like whether to arm the rebels in Syria."

Alongside the president, she vowed to take up such challenges in her new role.

"We have vital opportunities to seize and ongoing challenges to confront. We have much still to accomplish on behalf of the American people and I look forward to continue to serve on your national security team to keep our nation strong and safe," Rice said.

Richard Weitz, a security expert at the Hudson Institute, a Washington think tank, says Rice may have to temper her aggressive style to effectively broker between competing government agencies, a key function of the job.

"She fits the bill as far as being very close to the president. She's also a strong leader so will enforce discipline in the interagency process as well. [But] she'll have to perhaps be less outspoken than she was as the UN ambassador," said Weitz. "In this job, you're supposed to be more discreet."

Rice's appointment does not require Senate confirmation, a fact that may dampen potential protest from Republicans. Rice was considered the front-runner to become secretary of state last year before she became the focus of Republican ire over the administration's handling of the attacks on the U.S. mission in Benghazi, Libya.

Following the attacks on September 11, 2012, which killed U.S. Ambassador to Libya Christopher Stevens among others, Rice relied on incorrect talking points provided by the CIA in describing the incident as the result of a spontaneous protest.

It was later determined to be a planned terrorist assault.

Amid the controversy, Rice withdrew her candidacy for the post.

Obama also heaped praise on the outgoing Donilon, who has been in the job since October 2010.

The president said the 58-year-old had been decisive on initiatives ranging from ending the war in Iraq to transitioning in Afghanistan to tracking down former Al-Qaeda leader Osama bin Laden to signing the new START nuclear nonproliferation treaty with Russia.

Obama said he was "especially appreciative" to Donilon for engineering the U.S. pivot to the Asia-Pacific region.

One of Donilon's last big projects was helping to organize the unusual informal meeting between Obama and Chinese President Xi Jinping scheduled for June 7 at a California estate.

Donilon has said he planned to leave after Obama's first term but stayed on at the president's request until new secretaries of state and defense had settled in.

Obama also nominated Samantha Power, a human rights expert, Pulitzer Prize-winner, and former White House adviser, to take Susan Rice's place at the United Nations. She must be approved by the Senate.

With additional reporting by "The New York Times," AP, and Reuters