"And it's very hard to see how the administration expects diplomacy to succeed when at the same time it is designating partners in that diplomacy as terrorists."
George Bush, the US president, is under pressure from members of congress and members of his own administration, including his vice-president, Dick Cheney, who are frustrated by lack of progress in curbing Iran's nuclear ambitions.
Fighting force estimated to number between 125,000 and 350,000 outside regular Iranian military's chain of command
Has own navy, air force and special forces
Has strong influence over Iranian political life and widely involved in country's economy
Mahmoud Ahmadinejad, the Iranian president, rose through ranks of Guard and won presidency with support of its veterans' network.
The US military also says the Revolutionary Guards provides military support for anti-American fighters in Iraq.
The terrorist designation would allow the Bush administration to target the group's extensive business operations, including a crackdown on non-US companies doing business with the Guard's various financial enterprises.
The UN Security Council levied sanctions on specific Guard commanders and their businesses this year.
The Bush administration is expected to formally put the terrorist label on the Guard before the UN General Assembly meeting next month.
Washington is also pushing the security council to prepare another resolution against Iran over its refusal to give up sensitive uranium enrichment work - but it has had problems getting agreement from Russia and China.
It would be the first time the US has placed the armed forces of any sovereign government on its list of terrorist organisations.
The US list of individuals, businesses, charities and groups believed to be engaged in terrorist activities includes al-Qaeda; Hezbollah, the Lebanese Shia movement; and Hamas and Islamic Jihad, both Palestinian groups.