Russia, which also has supported Mr. Assad, favors Iran’s participation. But France’s foreign minister said in September that Iran should not be allowed to participate unless it first makes it clear that it would accept an outcome in which Mr. Assad would hand over power to a transitional body. Much of the Syrian opposition is also opposed to Iran having any role.

With a few weeks to go before the conference, one of the officials rated the chances of a Iran participating as “less likely than likely.”

The issue raises broad questions about how best to manage the West’s relations with Tehran. So far, the thaw in relations between the United States and Iran has been mainly limited to the November interim agreement suspending much of Iran’s nuclear program for six months. Technical talks on how to put that interim agreement into effect are still continuing, and it is unclear whether the agreement will be the basis for a more comprehensive accord to roll back Iran’s nuclear efforts.

Though American and Iranian officials have conferred at length on the nuclear question, they appear to have engaged in only very limited discussions of other regional issues.

With Western nations and Iran backing different sides in Syria, there have been no signs of the kind of political cooperation that was seen after the Taliban were ousted in Afghanistan in 2001.

The Obama administration has insisted that Mr. Assad must give up power and has provided limited support for moderate elements among the rebels who are trying to unseat him. By contrast, Iran has flown shipments of arms and members of its paramilitary Quds force to help Mr. Assad’s forces. Iran has also encouraged Hezbollah, the Lebanese militia, to intervene on Mr. Assad’s side.

At the same time, the Syria conflict has become a source of friction between the United States and its traditional Arab partners, especially Saudi Arabia, which is worried about Iran’s influence in the region. Those tensions became more pronounced after the Assad government used chemical weapons last year, and the White House shelved plans to mount a military strike in response. Instead, the United States worked with Russia to conclude an agreement to eliminate Syria’s arsenal of poison gas. That agreement was hailed by arms control experts as a breakthrough, but it appears to have left Mr. Assad firmly entrenched in power.