"If the opposition does not participate there will be no Geneva conference," Brahimi said in Damascus before returning to Beirut as part of a regional tour to garner support for the US-Russian peace initiative.
"The participation of the opposition is essential, necessary and important," the veteran Algerian diplomat said, adding the proposed conference was intended "to help the Syrians and resolve their problems".
Brahimi, who met with President Bashar al-Assad on Wednesday, said "the Syrian government has agreed to participate in Geneva".
"The opposition, whether the National Coalition or others, are trying to find a way to be represented," he said, referring to the main opposition bloc.
Brahimi also said he hoped the conference would be held "in the coming weeks, not next year”.
"I will return to Geneva on Tuesday, where I will meet the Russian and US delegations, and representatives of other countries might join us, in a bid to prepare the conference and to agree on a definitive date, which will be announced in the coming days."
The renewed bid for peace talks aimed at resolving the two and a half-year conflict comes after last month's US-Russian accord on dismantling Syria's chemical arsenal.
Syria's opposition has thus far baulked, with the National Coalition saying it will not take part if Assad's resignation is not on the table, a demand his government rejects.
Some militant brigades have warned any Syrians taking part in the talks would be viewed as traitors.
The Coalition, which is under pressure from its Western and Arab backers to attend Geneva II, is to meet on November 9 to decide on the talks.
In his remarks on Friday, Brahimi also called on the international community to help Syria and its parties to allow assistance through to besieged areas like Moadamiyet al-Sham.
"Children have not had any bread for nine months" in the rebel-held town southwest of Damascus, he said.
According to the envoy, "a third of Syria's inhabitants are directly affected" by the war.
Brahimi also raised the issue of the large number of prisoners in both government and militant groups’ hands. "These practices are unacceptable," he said.
NJF/NJF