Nikki Haley, the US ambassador to the United Nations, said Sunday Iran's government is "being tested by its own citizens" after three days of protests against the clerical regime.
"We pray that freedom and human rights will carry the day," she said in a statement.
Haley's comments echoed those of US President Donald Trump, who earlier Sunday said the United States was watching closely for human rights violations.
"Big protests in Iran. The people are finally getting wise as to how their money and wealth is being stolen and squandered on terrorism. Looks like they will not take it any longer," Trump tweeted.
Over the past year, Trump has held out Iran as Washington's top adversary in the Middle East, disavowing a 2015 accord that curbed its nuclear program and accusing it of destabilizing activities in the region.
At least two people have been killed and dozens arrested since protests broke out Thursday in the city of Masshad and spread to Tehran and other cities.
Iran's Interior Minister Abdolreza Rahmani Fazli on Sunday warned that protesters will "pay the price" for disrupting order.
Haley singled out Iran in a New Year's message that also denounced "oppressive governments" in North Korea, Venezuela and Cuba.
"The long-repressed Iranian people are now finding their voice," she said. "The Iranian government is being tested by its own citizens."