The United States has said it is "deeply troubled" by reports of violence and claims of polling fraud following the Iranian elections.
Ian Kelly, a US state department spokesman, said on Monday: "We're concerned about some of the treatment of demonstrators.
"We're calling for the Iranian authorities to respect the right of people to express themselves peacefully.
"We are deeply troubled by the reports of violent arrests and possible voting irregularities," he said.
He added Washington was closely watching the situation unfolding in Iran.
'Heartened by enthusiasm'
He said: "As we go forward, we are going to make our decisions based on US national interests and of course we're following the situation very closely.
"But ... we've made the decision to pursue direct diplomacy with Iran, particularly through the multilateral context."
He also said the US continued to be "heartened by the enthusiasm" of young people in Iran, many of whom staged protests against the re-election of Mahmoud Ahmadinejad, the Iranian president, in favour of rival Mir Hossein Mousavi.
Robert Gibbs, the White House press secretary, told journalists there were a "number of factors" over the election that gave the US "concern about what they had seen".
The US government's latest comments came after Joe Biden, the US vice-president, said on Sunday that he had "doubts" about the validity of the election.
"It sure looks like the way they're suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there's some real doubt," Biden told NBC television channel.
Official election results gave Ahmadinejad nearly 63 per cent of the vote, with Mousavi only on 34 per cent.
"That's what they're announcing. We have to accept that for the time being. But there's an awful lot of question about how this election was run," Biden said.
Biden said the US was going wait for "a thorough review" of the whole process before reacting further.
Hillary Rodham Clinton, the US secretary of state, said she hoped the outcome reflected the "genuine will and desire" of Iranian voters.
Clinton was responding to allegations by supporters of Mousavi that the outcome was rigged to give Ahmadinejad a decisive victory.
"We are monitoring the situation as it unfolds in Iran, but we, like the rest of the world, are waiting and watching to see what the Iranian people decide," Clinton said in Ontario, Canada.
So far Barack Obama, the US president, has not commented on the protests, saying only last Friday, when the Iran elections were held, that he was heartened to see "robust debate" over the vote.
Gordon Brown, the UK prime minister, said on Monday that the way the Iranian government responded to the protests would "have implications for Iran's relationships with the rest of the world in the future".
"The [Iranian] regime must address the serious questions which have been asked about the conduct of the Iranian elections," he told the UK parliament.
Both Germany and France summoned Iran's ambassadors to express concern over the use of force by police against demonstrators.
"The actions of the Iranian security forces are completely unacceptable," German Foreign Minister Frank-Walter Steinmeier said on public television.
After a meeting in Luxembourg, EU foreign ministers expressed "serious concern" at the weekend crackdown and called for an inquiry into the conduct of the election.
Ban Ki-moon, the UN secretary-general, said he had noted that Ayatollah Ali Khamenei, Iran's supreme leader, has ordered officials to look into the fraud allegations.
"I am closely following how this investigation will come out," he said, reiterating that "the genuine will of the Iranian people should be reflected and respected in the most transparent, fair and objective manner".