Barack Obama has accused prominent Republican senators of an "outrageous" attack on the character of Susan Rice, the US ambassador to the UN and potential nominee as secretary of state, in alleging a cover up over the Benghazi attack that killed the American ambassador to Libya, Chris Stevens.

The president denounced senators John McCain and Lindsey Graham for saying they would attempt to block Rice from leading the state department if Obama nominated her because she either lied or was incompetent in saying that the attack in Libya on September 11 was spontaneous and that there was no evidence of a link to al-Qaida.

Obama vigorously defended Rice at his press conference on Wednesday by saying that she was merely repeating the intelligence available at the time when she appeared on various Sunday television talk shows five days after the Benghazi attack.

"She made an appearance at the request of the White House in which she gave her best understanding of the intelligence that had been provided to her. If senator McCain and senator Graham and others want to go after somebody, they should go after me and, I'm happy to have that discussion with them," the president said. "But for them to go after the UN ambassador, who had nothing to do with Benghazi, and was simply making a presentation based on intelligence she had received, and to besmirch her reputation, is outrageous."

Obama said he was not prepared to comment on nominations to the State Department or other posts, but that he would not be deterred by threats from choosing Rice.

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