Under pressure from several quarters to harden his stance, President Barack Obama condemned the regime in Tehran for the violence against protesters, but still gave no indication that he was ready to step back from his campaign promise to reach out to the country.
The "world is watching," Mr Obama told the ruling clergy in his sternest statement yet. "We mourn each and every innocent life that is lost.
"We call on the Iranian government to stop all violent and unjust actions against its own people."
Reports that at least 10 people had been killed on Saturday in a government crackdown further underlined the difficulty of Mr Obama's position. Already, the turmoil has complicated Mr Obama's attempts to draw Iran into more constructive talks with the West on its nuclear programme. Making any such diplomatic overtures to a regime sullied by the events of recent days now looks less tenable.
One leading voice on foreign affairs on Capitol Hill, Senator Richard Lugar of Indiana, yesterday suggested that new contacts should still be pursued with Iran, adding that either Mr Obama or Secretary of State Hillary Clinton should be ready to meet their counterparts from Iran.
Some analysts see an opening for the US over the longer term from the turmoil in Iran if it has the effect of loosening the unquestionable authority of the ruling theocracy.
The fine line walked by Mr Obama over the last several days may already be paying off. It has not gone unnoticed in Washington that just as several European Union leaders were faster to lambaste Iran for the conduct of the election and repression of the protests, so some of those countries have been the targets of retaliatory rhetoric, among them Britain especially.
Nevertheless, Mr Ahmadinejad yesterday included the US alongside the UK in a new broadside against claimed foreign interference, warning that "hasty comments" would mean neither country would "have a place in the circle of the Iranian nation's friends".
There are growing divides within Washington over the wisdom of Mr Obama's approach. An influential member of the US Senate, Republican Lindsey Graham of South Carolina, said the President was too "timid and passive".
But Mr Obama said the Iranian people will ultimately judge the actions of their own government. "If the Iranian government seeks the respect of the international community, it must respect the dignity of its own people and govern through consent, not coercion."