George Bush, the US president, was briefed on the contents of what was the first publicly announced personal communication from an Iranian president to his US counterpart since the 1979 Islamic revolution and the severance of diplomatic ties in 1980.
Condoleezza Rice, the US secretary of state, dismissed the letter from Mahmoud Ahmadinejad, saying it was not a diplomatic opening and just covered history, philosophy and religion.
"This letter isn't it. This letter is not the place that one would find an opening to engage on the nuclear issue or anything of the sort," she said.
"It isn't addressing the issues that we're dealing with in a concrete way."
She would not discuss the contents in detail but made it clear that the United States would not change tack on Iran.
Other sceptical US officials said the letter may have been timed to influence a UN Security Council debate.
Washington seeks support from allies for tougher action on Tehran but faces resistance to sanctions from China and Russia.
John Negroponte, the US director of national intelligence, said it was necessary to examine "whether and in what way the timing of the dispatch of that letter is connected with trying in some manner to influence the debate before the Security Council".
The White House confirmed that the letter had been received after it was sent via the embassy of Switzerland, which acts as a go-between for the US and Iran.
John Bolton, the US ambassador to the UN, said he had read the letter.
"I think it is typical of Iran that when major decisions are about to be taken ... that they have tried to throw sand in the eyes of the proponents of the action," he said. "That's what this may be."