Still, the delegates will not be particularly representative, Mr. Brahimi remarked to reporters after his meetings. The conference, which will start in the Swiss town of Montreux and then move to Geneva, will mark only the start of a process, he added, expressing the vague hope that in later stages of negotiations “the representation of the people of Syria will be better and better.”

Friday’s discussions also failed to overcome the American view that participation by Iran, a crucial regional ally of President Bashar al-Assad, would not be the right thing to do, Mr. Brahimi said.

The United Nations and Mr. Brahimi have long advocated a role for Iran as a regional power, and last month’s breakthrough accord between Iran and world powers on curbing its nuclear program looked as if it might lower objections to accepting Iran’s involvement in the conference. As Ms. Sherman met Mr. Brahimi on Friday, American experts met Iranian officials across the city to work out details of carrying out that accord.

But the two issues are entirely separate, the senior American official said. Iran’s failure to endorse an earlier Geneva communiqué that provides the basis for the conference and calls for the creation of a transitional government by mutual consent remains one sticking point, the official said.

Iran should also think about withdrawing the military personnel that it has sent to Syria and ending its support for Lebanon’s pro-Assad Hezbollah militia, which has also sent fighters to buttress the Syrian Army, the official said.

If Iran is not present, “we still would like to work with them,” said Mr. Brahimi, who has had regular contacts with the Iranian foreign minister, Mohammad Javad Zarif, in recent weeks. American officials agreed that Tehran could engage in the process in a variety of ways without taking part in the conference.

The more troubling issue, Mr. Brahimi made clear, is developments on the ground in Syria. “The fighting is intensifying all the time and aid that is available is not reaching the people who need it,” he said.

“We hope that now we have a date for the conference, the parties will take a number of unilateral decisions as measures to indicate they are coming to Geneva to end this conflict,” he added. He called on both sides to release prisoners, especially women and children, and urged better access for humanitarian aid and an end to the use of “devastating weapons,” referring to the government’s barrel bombing of the northern city of Aleppo in recent days.