By Paul Reynolds
World affairs correspondent, BBC News website
The latest American move against Iran has again raised the issue of whether this is simply the hardening of sanctions against Iran or whether it is the prelude to a military strike.
The new sanctions build on previous ones, and more may be planned
Washington has designated the "Quds" force (Quds being the Arabic name for Jerusalem) within the Iranian Revolutionary Guards as a "supporter of terrorism".
The Guards as a whole have also been accused of proliferating weapons of mass destruction, a reference to ballistic missile development in which they are allegedly involved.
The Bush administration wants to turn the heat up on Iran and its President Mahmoud Ahmadinejad, who has just signalled his own hard line by replacing Iran's main nuclear negotiator with one of his own associates.
The aim of this move, which has been under discussion since the summer, is to send a message to Iran to change both its nuclear and missile development and to stop the support the US says it gives to the Shia militias in Iraq that have attacked US and UK forces there.
Iran says its nuclear programme is entirely peaceful and denies supporting the militias with lethal technology.
The measures are intended to squeeze the international operations of the Guards' many commercial activities. Among these are roles in managing Tehran's airport and underground transport systems.
Two UN Security Council resolutions, in December and March, sought to target Iranian trade in materiel and equipment connected with its nuclear and ballistic missile programmes and also named three aviation companies run by the Revolutionary Guards as well as seven of its officers.
This new American restriction will build on these resolutions. It will add to the pressure on US allies and business partners to curtail their dealings with Iran.
However, whether it leads to a change in policy by the present Iranian government must be doubted. The US has imposed wide-ranging trade sanctions on Iran since 1979 and these have made no difference. The hope is that in due course there will be a change of government in Iran.
The US will also continue to try to get a further Security Council resolution tightening and extending sanctions on Iran if it does not suspend its enrichment of uranium. Russia and China have blocked this but talks will resume in November. Russia has said there is no evidence that Iran is building a nuclear bomb.
However, the existing pressure has clearly not been enough. Iran is still defying the Security Council demand for it to suspend uranium enrichment to allow talks about its future nuclear plans to be discussed.
In the meantime, the drumbeat of American accusations against Iran is growing. US Secretary of State Condoleezza Rice said on Monday: "The policies of Iran constitute perhaps the single greatest challenge for American security interests in the Middle East."
The unanswered question is whether the new American move is another step on a path to a military strike against Iran. Vice-President Dick Cheney said last Sunday: "Our country and the entire international community cannot stand by as a terror-supporting state fulfils its most aggressive ambitions."
Others suggest that Ms Rice prefers toughening up the diplomatic approach, which is why she is supporting this "terrorist" designation. And the new chairman of the joint chiefs of staff, Admiral Mike Mullen, warned this week that a war in a third Muslim nation (after Iraq and Afghanistan) would have "extraordinary challenges and risks associated with it".
Role of corps
The Revolutionary Guard Corps (known as the Pasdaran) was formed after the Iranian
revolution in 1979 and then took a major role during the war launched against Iran by Saddam Hussein, during which it developed the concept of the human-wave attack.
It forms a significant but separate part of the Iranian armed forces, with internal security and border protection duties (its forces captured the 15 British sailors and marines in the Gulf earlier this year). But it also operates Iran's ballistic missiles and is believed to have a role in the nuclear field as well.
The US and Israel accuse it of arming Hezbollah in Lebanon as well as the Shia in Iraq.
Iran's President Mahmoud Ahmadinejad was once a member, so a move against the Guards as a whole would also be seen as a move against him.