US President Barack Obama has nominated Samantha Power, an Irish-born former White House aide, to replace Susan Rice as US ambassador to the United Nations.
The move comes as Mr Obama shifts his foreign policy team to include Ms Rice as his national security adviser amid the resignation of his current top security aide, Tom Donilon.
Ms Power is a long-time adviser to President Obama who worked on his 2008 presidential campaign and ran the human rights office in the White House.
She left the administration in February, but was considered the favourite to replace Ms Rice at the UN.
Ms Power's selection for the UN post has been described as surprise by some White House observers.
UN and US diplomats had anticipated that Mr Obama would choose Deputy Secretary of State Williams Burns.
This is a fresh chance for Ms Power after her discretion and diplomatic skills were called into question when she labelled then-Democratic presidential candidate Hillary Clinton a "monster" in 2008.
The remark prompted her resignation from Obama's campaign team.
Her background as an outspoken defender of human rights is likely to make her a strong voice in the administration for a stronger US role in protecting human rights in such places as Syria, China and Sudan.
Ms Power won the 2003 Pulitzer Prize in general nonfiction for her book "A Problem from Hell: America and the Age of Genocide," which examined US foreign policy toward genocide in the 20th century.
She is a graduate of Yale University and Harvard Law School.
According to a biography on the White House website, Ms Power also served as a professor at Harvard's John F Kennedy School of Government, where she taught courses on US foreign policy, human rights, and extremism.
