US special representative for Iran Brian Hook on Thursday said the Iranian regime might have killed more than 1,000 protesters and arrested nearly 7,000 since demonstrations broke out last month.
“As the truth is trickling out of Iran, it appears the regime could have murdered over 1,000 Iranian citizens since the protests began," Mr Hook said at the State Department.
He did not reveal the source for this number but said it might include “at least dozen children”.
Amnesty International estimated on Monday that 208 Iranian protesters had been killed and The New York Times said between 180 and 450.
Mr Hook said the US obtained a video showing that in the city of Mahshahr alone, Tehran's Revolutionary Guard Corps “murdered 100 Iranians, possibly more”.
He said that those killed were taken to “an unknown location”.
Mr Hook said that in response, the US was sanctioning two Iranian prisons where thousands of protesters may have been kept – the Greater Tehran Central Penitentiary and Qarchak.
He confirmed that the US intercepted a cache of Iranian weapons, including anti-ship and anti-tank missiles, heading to Yemen last week for delivery to the Houthi rebels.
"On November 25, a US warship conducted a flag-verification boarding in international waters off the coast of Yemen," Mr Hook said.
"We interdicted a significant hoard of weapons and missile parts, evidently of Iranian origin."
He accused Iran of trying to prolong the war in Yemen, and said “Iran does not speak for the Houthis”, after their ceasefire gesture to Saudi Arabia in September.
The US and Saudi Arabia have been in direct talks with the Houthis, senior diplomatic sources say.