"It is extremely difficult to bring people who have been killing one another for two years just by a magic wand to a conference like this. It will take time, but I hope it will happen," Brahimi told reporters on the sidelines of an event in Washington on Monday.
"There are still issues that have not been solved. We are hopeful. That's all we can say."
The talks, dubbed Geneva 2, were initially planned for late May after the idea was first floated during a Moscow meeting between US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov.
Amid difficulties on agreeing who should sit at the table, the calendar slipped into June, and then July, with the best estimates now hoping for a date in September.
There is also disagreement between the United States and Russia about whether Iran should attend the talks, which are aimed at bringing together the opposition and the Syrian government to find a political solution to end a conflict that is now in its third year.
"The United Nations have made it very clear that we would like all countries... with interests and/or influence (to) attend Geneva, and that includes Iran," Brahimi told the reporters.
And amid debate in Washington about arming the Syrian opposition, Brahimi said the UN stand was also very clear.
"Arms don’t make peace. We would like to see the delivery of arms stopped to all sides."
Another major problem is divisions between the Syrian opposition, Brahimi earlier told an audience gathered at the Carnegie Endowment for International Peace.
"The opposition is divided that's no secret. They are trying to get their act together and work their way to being a truly representative organization," he said.
But he sought to allay concerns that any transitional government in Syria could include Assad.
"The time for cosmetic changes in Syria and elsewhere is past and people are demanding a transformation of their society."
He also took issue with perceptions that the Syrian government seemed to be gaining the upper hand. "Most people in November last year were convinced that the regime had lost ... I think that was not correct," he said.
"Now you have a lot of people saying that the Syrian government is winning. It is doing much better than it did in November last year, that is true. But in situations like this winning and progress are two different things," Brahimi added.
The conflict in Syria started in March 2011, when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.
The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.
As the foreign-backed insurgency in Syria continues without an end in sight, the US government has boosted its political and military support to Takfiri extremists.
Washington has remained indifferent about warnings by Russia and other world powers about the consequences of arming militant groups.