Listing would set in motion a series of automatic sanctions that would make it easier for the United States to block financial accounts and other assets controlled by the guard. In particular, the action would freeze any assets the guard has in the United States, although it is unlikely that the guard maintains much in the way of assets in American banks or other institutions.

In the internal debate over American policy toward Iran, Ms. Rice has succeeded over the last year in holding the Bush administration to a diplomatic course in which America and five other world powers have used the Security Council to impose sanctions to try to get Tehran to suspend its enrichment of uranium.

Image Ayatollah Ali Khamenei, Irans supreme leader, at a ceremony with Revolutionary Guard members in Tehran in November. Credit... Mohammad Berno/Document Iran, via Atlas Press

But in recent months, there has been resurgent debate within the administration about whether the diplomatic path is working, with aides to Vice President Dick Cheney said to be among those pushing for greater consideration of military options. The debate has been kindled by reports from international inspectors detailing Iran’s progress in its nuclear program, including the installation of more than 1,000 centrifuges to enrich uranium, as well as the assertions from American intelligence officials about an Iranian role in providing arms and other support to Shiite militias in Iraq and to Taliban militants in Afghanistan.

Iran has repeatedly denied that it is seeking to build nuclear weapons, that it is helping in any way to facilitate attacks on American troops in Iraq or that it is shipping any weapons to the Taliban, a group Iran opposed in the 1990s.

On Tuesday, President Mahmoud Ahmadinejad again dismissed American complaints that Iran is providing weapons to the Taliban. Speaking in Kabul, Afghanistan, after talks with President Hamid Karzai, he said Iran was “fully supporting” its new government.

Mr. Karzai played down the dispute over the weapons shipments, as he did during a visit to the White House this month. He said that Afghanistan and Iran were “brothers” and that both the United States and Iran were helping reconstruct his country.