Iranian security forces may have killed more than 1,000 people since protests over gasoline price hikes began in mid-November, according to US Special Representative for Iran Brian Hook.
Among those were at least a dozen children, Hook said, but cautioned that the numbers were not definitive as Tehran blocked information.
He said that "many thousands of Iranians" had also been wounded and at least 7,000 detained in Iran's prisons.
The unrest, which began on November 15 after the Iranian government abruptly raised fuel prices by as much as 300 per cent, spread to more than 100 cities and towns and turned political as young and working-class protesters demanded clerical leaders step down.
"As the truth is trickling out of Iran, it appears the regime could have murdered over a thousand Iranian citizens since the protests began," Hook told reporters at a briefing at the State Department on Thursday.
Among the dead were as many as 100 protesters killed in an incident last month in a city in south-western Iran, the State Department's Hook said.
The department said it received a video showing members of the Islamic Revolutionary Guards Corps (IRGC) spraying fleeing protesters with machine gun fire, after demonstrators blocked a road.
"Without warning IRGC tracked them down and opened fire. Between the rounds of machine gun fire the screams of the victims can be heard," Hook said at a briefing.
Tehran has given no official death toll but Amnesty International said on Monday it had documented the deaths of at least 208 protesters, making the disturbances the bloodiest since the 1979 Islamic Revolution.
Tehran's clerical rulers have blamed "thugs" linked to its opponents in exile and the country's main foreign foes, the United States, Israel and Saudi Arabia, for the unrest.
