Rice said she did not expect "direct engagement" between the two leaders when Mr. Obama and Ahmadinejad are in New York for the opening of the U.N. General Assembly.
As the Associated Press reports, Ahmadinejad today criticized the West and Israel and suggested the Holocaust may not have happened. Rice called the comments "hateful."
White House spokesman Robert Gibbs also condemned the comments, calling them "baseless, ignorant and hateful." He suggested they would only serve to further isolate the country.
Iranian protesters faced a violent crackdown following Ahmadinejad's reelection, which they suggested was illegitimate, and the United States has criticized Iranian for illicit weapons programs. Mr. Obama has stated he wants his administration to engage with the country's leaders.
Both the president and Ahmadinejad speak to the General Assembly Wednesday.
Mr. Obama is planning to have one-on-one meetings at the U.N. with President Hu of China, Japanese Prime Minister Hatoyama and President Medvedev of Russia, the White House said.