WASHINGTON (Reuters) - United States President Barack Obama plans to appoint US Ambassador to the United Nations Susan Rice as his national security adviser, replacing Mr Tom Donilon, who is resigning, a White House official said on Wednesday, in a major change in his foreign policy team.

Mr Obama plans to make the appointment, first reported by the New York Times, later on Wednesday.

He also will nominate Ms Samantha Power - a Pulitzer Prize-winning author, former White House aide and Harvard professor - as UN ambassador, the White House official said.

Ms Rice's appointment as Mr Obama's top security adviser does not require Senate confirmation.

Mr Obama had wanted to name Ms Rice as his secretary of state, but she ran into heavy opposition from Republicans over remarks she made in the aftermath of the Sept 11, 2012, attack on the US mission in Benghazi, Libya, that killed four Americans, including ambassador Christopher Stevens.

Ms Rice, a close confidante of Mr Obama, withdrew her name from consideration for the top diplomatic post last December.

Mr Donilon, who served for more than four years as Mr Obama's top national security adviser, will leave in early July, a White House official said.

The shake-up comes as the Obama administration struggles with its response to the civil war in Syria.