WASHINGTON (Reuters) - Susan Rice withdrew her name from consideration as U.S. secretary of state on Thursday in the face of what promised to be a difficult Senate confirmation battle.

Rice, the U.S. ambassador to the United Nations and a close confidante of President Barack Obama, said she was withdrawing from the process to avoid a lengthy, costly and disruptive confirmation battle.

"That trade-off is simply not worth it to our country," she wrote in a letter to Obama.

Rice has faced questions about comments she made days after the September 11 assault on the U.S. mission in Benghazi, Libya, when the Obama administration said the attack was a result of a spontaneous demonstration over a film made in the United States that was insulting to the Prophet Mohammad.

Obama issued a statement saying he had accepted her decision and is grateful she will continue as the U.S. ambassador to the United Nations.

Hillary Clinton is planning to step down as secretary of state.

(Reporting By Steve Holland and Jeff Mason; Editing by Will Dunham)