The US has issued sanctions on a general in Iran's military wing, a Syria-based TV station and three individuals its accuses of promoting violence in Iraq.
The sanctions, announced on Wednesday, come amid increasing tensions between Tehran and Washington after Iranian speedboats confronted US navy ships in the Strait of Hormuz.
George Bush, the US president, called the Iranian action "provocative".
The US treasury department said that Ahmed Foruzandeh, a general in the Qods force, "leads terrorist operations" against US forces in Iraq and has directed assassinations of Iraqi figures.
Last October the US said Iran's Revolutionary Guard Corps proliferated weapons of mass destruction and imposed sanctions on its Qods force.
The US also accuses the Qods force of arming and training anti-multinational forces in Iraq.
Simultaneously, economic sanctions were placed on Al-Zawra television channel which the US accuses of receiving money from al-Qaida.
Iraqis sanctioned
The three sanctioned Iraqis, who are living in exile in Iran and Syria, are named as Mishan al-Jaburi, Ismail Hafiz al-Lami and Abu Mustafa al-Sheibani.
The sanctions prohibit all transactions between those designated and any US person, and freeze all assets they may have under US jurisdiction.
"Iran trains, funds and provides weapons to violent Shia extremist groups while Syria provides safe haven to Sunni insurgents and financiers," Stuart Levey, the treasury's undersecretary for terrorism and financial intelligence, said in a statement.
"Today's action brings to light the lethal action of these individuals and we call on the international community to stand with us in isolating them from the global economy."
Both Iran and Syria strongly deny stirring up violence in Iraq and say the US is to blame.