Iran earlier hanged two men convicted of being Mohareb (enemies of God), in the first executions of dissidents since protests erupted over the disputed presidential election in June, news reports said.
"The US strongly condemns these unjust executions," deputy White House spokesman Bill Burton said aboard Air Force One.
"We see it as a low point in the Islamic Republic's unjust and ruthless crackdown of peaceful dissent.
"Murdering political prisoners who are exercising their universal rights will not bring the respect and legitimacy the Islamic Republic seeks.
"It will only serve to further isolate Iran's government in the world and from its people."
Mohammad Reza Ali Zamani and Arash Rahmani Pour were also charged with plotting to topple the Islamic regime, the ISNA news agency said.
State Department spokesman Philip Crowley said that from the US vantage point, "it's unclear that these individuals had anything to do with the turmoil surrounding the elections last year."
President Barack Obama's administration has extended a hand of dialogue to Iran to repair three decades of sour relations since the 1979 Islamic revolution.
But the administration has been increasingly frustrated at what it sees as President Mahmoud Ahmadinejad's reluctance to address allegations that his hardline administration is building nuclear weapons.
Crowley said that the United States would continue to support a diplomatic resolution with Iran but also speak out against its "increasingly ruthless repression."
"We pursue engagement with Iran and diplomacy because it is in our national interest to do so," Crowley told reporters.
"We have an outstretched hand to Iran because we want to see Iran play a constructive role in the region. We want to see Iran have a different kind of relationship with its own people," he said.
