The United States’ top diplomat for Iran has slammed the Islamic Republic for reports about sexual assaults on protest detainees, saying it is “unspeakable”.

Robert Malley, the US special envoy for Iran, said in a tweet Tuesday that a report by CNN describes “unspeakable acts of sexual violence by Iranian officials in detention centers. It’s a reminder of what is at stake for the Iranian people - and of the lengths to which the regime will go in its futile attempt to silence dissent.”

In an investigation published by CNN on Monday, covert testimony revealed sexual violence against protesters, including boys, in Iran’s detention centers since the start of the unrest.

In some of the cases CNN uncovered, the sexual assault was filmed and used to blackmail protesters into silence, according to sources who spoke to the victims.

In reaction to this report, State Department spokesperson Ned Price also harshly criticized the Iranian regime, saying that Washington continues to hold the perpetrators accountable.

Price said Tuesday that the US “is disgusted by the reports and eye-witness accounts of protestors, including minors, being sexually assaulted while in the custody of law enforcement.”

“Iranian authorities' use of sexual violence as a tool for protest suppression brings into stark contrast the bravery of Iran's protestors in the face of such reprehensible measures to intimidate them and end the protests,” added Price.

Condemning the brutality of the clerical regime, Price added that it appears no act is beneath the Islamic Republic's leadership in their attempts to silence dissent.