Barack Obama, the US president, has so far been careful not to make strong comments on the legitimacy of the June 12 vote.
However, Washington has condemned Tehran for its use of violence against opposition demonstrators who were questioning the veracity of the poll result.
Dozens of people are thought to have been killed and about 2,000 detained during the unrest.
On Tuesday, Washington called for the release of Kian Tajbakhsh, an Iranian-American scholar, who has been put on trial with about 100 other defendants over his alleged role in the post-election violence.
"We are deeply concerned of reports that Iranian-American scholar Kian Tajbakhsh was recently charged by an Iranian court without the benefit of a lawyer," Robert Wood, the deputy spokesman for US state department, said.
"Given that the charges facing Mr Tajbakhsh are without foundation, we call on Iran's leadership to release Mr Tajbakhsh without delay."
Obama and the leaders of France, Britain and Germany have refused to congratulate Ahmadinejad on his re-election.
"In view of the circumstances of the controversial re-election, the chancellor will not, as usual, write the normal letter of congratulation," a German government spokesman said.
The British government has said it will send its ambassador to Ahmadinejad's swearing-in on Tuesday, but would also refuse to formally
"The prime minister will not be writing to congratulate Mr Ahmadinejad," a spokesman for Gordon Brown, the British prime minister, said.
The Iranian government has insisted that the election was free and fair and has accused Britain and the United States of stoking the post-election demonstrations, a charge those countries have denied.