The United States today imposed sanctions on an Iranian general from the elite Qods force as well as three Iraqis living in exile for fomenting violence in their home country.
The US Treasury Department, which made the announcement, named the general as Ahmed Foruzandeh, who it said "leads terrorist operations" against US forces in Iraq and directed assassinations of Iraqi figures.
In addition, sanctions were also put on Syrian-based Al-Zawra television station and three Iraqi exiles now believed to be in Syria or Iran.
In October, the United States designated the Revolutionary Guard Corps a proliferator of weapons of mass destruction and its elite Qods force as a supporter of terrorism.
Tensions between Iran and US heightened this week after it was claimed three US navy ships were threatened Iranian boats in the Strait of Hormuz at the weekend.
Washington claims Iranian boats aggressively approached three US naval ships in the waterway, a major oil shipping route off Iran's coast, and threatened that the ships would explode.
French Foreign Minister Bernard Kouchner today described Iran's behaviour as very dangerous and warned against any future acts of "provocation".
The US says the incident almost led to an exchange of fire, and President George W. Bush yesterday said it was "a provocative act".
"I describe Iran's behaviour as very dangerous. I call on them to show restraint," Mr Kouchner told a news conference when asked about the incident.
The United States released a video of the weekend incident, including a recording of what it said was the exchange between the two sides. Iran rejected the footage as fake and accused Washington of trying to stir up tension in the region.
Mr Kouchner sparked a diplomatic storm last year when he said in a live television interview that the world should prepare for the possibility of war with Iran over its nuclear programme, but he has since said he is doing everything to avoid that prospect.
French President Nicolas Sarkozy, however, said in a magazine interview last month that there is a real risk of war with Iran and Israel, rather than the US, was the country most likely to launch an attack on the Islamic Republic.