The UN-Arab League envoy to Syria has warned that there can be no peace talks without the opposition.
"We will say it's happened only when it happens," he told reporters at a press conference, acknowledging that the Geneva gathering cannot take place if the opposition refuses to take part.
In a possible sign the Damascus meetings had not gone too well, Syrian Information Minister Omran al-Zoubi slammed Mr Brahimi for equating between the opposition and the government in the press conference.
"Brahimi ... wants to satisfy all parties at the expense of truth and the Syrians alike," Mr al-Zoubi told Al-Mayadeen TV, adding that Mr Brahimi should be a neutral and evenhanded broker.
Mr Brahimi's plea came as Syrian state TV and opposition activists said the army captured a strategic town in the country's north believed to be the home of a chemical weapons production facility and storage sites.
Safira has been the scene of three weeks of intense fighting as the army kept trying to retake the town from rebels who have been in control there for more than a year. Safira is believed to be one of two sites that chemical weapons inspectors were unable to visit because of security concerns.
The town is also strategic as a supply route for Syrian government forces in the contested city of Aleppo. Syrian activists in Aleppo province confirmed the rebels had withdrawn from Safira overnight under heavy fire, leaving it to government troops.
The capture came just hours after officials said Israeli warplanes had attacked a shipment of Russian missiles inside a Syrian government stronghold - a development that threatened to add another volatile layer to regional tensions from the Syrian conflict.
The US and Russia have been pushing for a peace conference that would bring both sides of the Syrian conflict to the table in Geneva later this month.
More than 120,000 people have been killed so far in the war, now in its third year, according to the Syrian Observatory for Human Rights, a Britain-based watchdog. Millions of Syrians have been uprooted from their homes because of the fighting.
Mr Brahimi warned that if the crisis goes on, expectations are that those directly affected by the crisis may reach half of Syria's total pre-war population of 23 million people.
"It is time for Syrians to co-operate and for others in the region and outside to cooperate with them to end this crisis," Mr Brahimi said.
The envoy, who this week met Mr Assad and Damascus-based opposition groups, said the Syrian government has confirmed it would attend.
The deeply fractured Syrian opposition groups are split on whether to attend. They also disagree over conditions for taking part - from demands that Mr Assad step down right away to guarantees that he would not be part of a negotiated solution for the country's future
This time, Brahimi appeared to put the onus on the opposition, saying talks in Geneva cannot "go forward without the opposition."
"The participation of the opposition is essential, necessary and important," he said.
The opposition is made up of different factions, many of them politicians based in exile - the majority of whom are part of the main umbrella group, the Syrian National Coalition. There are also the Damascus-based opposition groups. But even within the Coalition, there are deep splits.
Mr Assad himself has said he will not talk to members of the Coalition whom he considers agents of the West, nor with armed rebels. The rebels themselves are a mix of various groups, from the mainstream Free Syrian Army to the extremist but powerful and effective al Qaida-linked groups.