The United States said Monday it was "deeply troubled" by the escalating violence in Iran which has flared since the disputed election victory of hardline President Mahmoud Ahmadinejad, AFP reported.
The State Department and the White House both expressed concern about claims of irregularities, but stopped short of branding the elections as fraudulent, although they did call for the rights of free expression to be respected.
"We are deeply troubled by the reports of violent arrests and possible voting irregularities," said State Department spokesman Ian Kelly.
"We're concerned about some of the treatment of demonstrators, and we're calling for the Iranian authorities to respect the right of people to express themselves peacefully."
He added Washington was closely watching the situation unfolding in Iran.
Aboard Air Force One, as President Barack Obama headed to Chicago for a speech on healthcare, White House spokesman Robert Gibbs also expressed disquiet about the election results.
"Obviously we continue to have concern about what we've seen. Obviously the Iranians are looking into this, as well," Gibbs said.
He also appeared to hint that whatever the ultimate outcome of the post-election political showdown in Tehran, Washington was still committed to a diplomatic effort to thaw frozen US-Iranian relations.
"I think what's important is the concerns that we have about their nuclear weapons program, and the concern we have about their support for terror isn't any different than it was on Friday."
The Obama administration has offered Iran unconditional talks in a bid to ease decades of tensions and mutual mistrust.
The US reaction echoed the concerns being expressed in Europe on Monday, as French President Nicolas Sarkozy called for "full light" to be shed on the vote results.
British Prime Minister Gordon Brown said Iran must answer "serious questions" about the vote and said the way Tehran handles the post-election crisis could determine its relations with the international community.
The top US officials spoke after a protestor was reportedly shot dead during clashes in Tehran as massive crowds of people defied a ban to stage a rally against Ahmadinejad's re-election.
A local photographer said the protestor had been shot with a bullet to the head and that several more were wounded when violence erupted outside a local base of the Islamic Basji militia, which had been set ablaze.
People were seen fleeing the area as police fired tear gas on protestors, an AFP correspondent said, as huge clouds of smoke billowed into the sky.
The violence flared after Ahmadinejad's defeated rival Mir Hossein Mousavi appeared in public for the first time since an election that has sharply divided the nation and triggered a wave of protests and rioting.