U.S. authorities have imposed sanctions on two Iranian security commanders for collaborating with the Syrian government in cracking down on protesters in Syria.

AFP reports that Brigadier General Esmail Ahmadi Moghaddam, the head of Iranian security forces, and his deputy, Ahmadreza Radan, have been added to the U.S. blacklist.

The U.S. Treasury Department said Radan had travelled to Damascus in April to give expert assistance to Beshar Assad’s government on putting down the opposition.

While announcing sanctions against the Syrian Political Security Directorate, the U.S. Treasury’s acting Undersecretary for Terrorism and Financial Intelligence, David Cohen said: “Also today we are further exposing Iran’s provision of military and security forces to support the Syrian government’s ongoing violence and repression of the Syrian people.”

Syrian authorities deny receiving any assistance from Iran in dealing with unrest.

Earlier, the United States had accused Qods branch of the Revolutionary Guards of aiding the Beshar Assad government in quashing the protests, but Iran has denied all such assertions.

Last week, the European Union blacklisted three officials for their involvement in oppressing Syrian protesters: the head of Iran’s Revolutionary Guards Corps (IRGC), Mohammad Ali Jafari; Ghassem Soleymani, the head of the IRGC Qods branch; and Hossein Taeb, the head of IRGC intelligence.

The Syrian government has been dealing with protests since last March. Beshar Assad has called the protests a conspiracy against national unity. An estimated 1,600 people reportedly have been killed since the unrest began.