"It appears the regime could have murdered over 1,000 Iranian citizens since the protests began," Efe news quoted State Department Special Representative for Iran, Brian Hook, as saying on Thursday.
Hook, however, said he was not be certain of that figure because "the regime blocks information".
According to Hook, in one single protest in the city of Mahshahr, more than a hundred people died and when it was over, the bodies were loaded in trucks.
"We do not yet know where these bodies were taken, but we are learning more and more about how the Iranian regime treats its own people," he added.
In response, Iran's judicial spokesperson Gholamhossein Esmaili said: "The numbers and figures that are being given by hostile groups are utter lies and the statistics have serious differences with what they announced."
Protests broke out in Iran on November 15 over rising oil prices and its rationing, and led to criticism of the country's theocratic system.
US President Donald Trump also mentioned the protests while speaking to the media on Thursday.
The Iranian regime "has killed hundreds and hundreds of people in a very short period of time. They're killing protesters. They turned down their internet system. People aren't hearing what's going", he added.
Days earlier, Trump accused Iran of killing "thousands and thousands" of people.
( With inputs from IANS )