Since he came to office in January, Barack Obama, the US president, has stated that he would like to engage in dialogue with Iran, but on Saturday he demanded an end to the crackdown on the protests.
"We call on the Iranian government to stop all violent and unjust actions against its own people," he said in a written statement.
"The universal rights to assembly and free speech must be respected, and the United States stands with all who seek to exercise those rights."
Manouchehr Mottaki, Iran's foreign minister, earlier said that Britain had "plotted against the presidential election for more than two years" and accused the country of organising an "influx" of agents into Iran before the polls.
"Elements linked to the British secret service were flying in in droves," he said.
But David Miliband, Britain's foreign minister, dismissed claims that his country had been providing support to the tens of thousands of people who have taken to the streets.
"I reject categorically the idea that the protesters in Iran are manipulated or motivated by foreign countries," he said.
"Foreign minister Mottaki's attempt in remarks to diplomats in Tehran this morning to turn the dispute among Iranians about the election results into a battle between Iran and other countries - the UK in particular - is without foundation."
Mottaki also rebuked Germany and France for raising questions about claims of voting irregularities during the election.
Germany called on Sunday for all the votes to recounted, protests to be allowed to take place unhindered and the media to be able to report without restriction, but did not address Mottaki's comments directly.
"Germany stands on the side of the people in Iran who want to exercise their right to freedom of expression and freedom of assembly,'' Angela Merkel, the German Chancellor, said in a brief statement.Dialogue urged
Bernard Kouchner, the French foreign minister, urged the international community to pursue dialogue with Iran's leaders, according to a French newspaper report.
He was quoted by Journal du Dimanche as saying that while Iran's protesters are calling for international support, "this doesn't mean we will settle the problem in place of the Iranians".
Kouchner also hailed the Iranian protesters' "courage to show their thirst for openness, freedom and their demand for electoral transparency".
Ali Larijani, Iran's parliamentary speaker, has suggested that ties with Britain, France and Germany should be reconsidered in view of their "shameful" statements on the vote, according to state radio.