A second round of peace talks between the Syrian government and the opposition got bogged down quickly over who was responsible for escalating violence that has killed hundreds in the past few days and disrupted food aid for trapped civilians.
The first face-to-face meetings adjourned 10 days ago having achieved little beyond getting the warring sides into the same room. This time, the two appeared even farther apart - with no immediate plans to even sit at the same table.
"The negotiations cannot continue while the regime is stepping up its violence against the Syrian people," opposition spokesman Louay Safi told reporters following a 90-minute meeting with Mr Brahimi. "It is not acceptable that the regime will send its own delegation to talk peace while it is killing our people in Syria. This must stop. We asked the international community to do something about it."
The opposition insists the talks' aim is to agree on a transitional governing body that would replace president Bashar Assad. The government delegation wants to focus on halting "terrorism", a reference to the rebels fighting to topple Assad.
But the increased violence on the ground provided ammunition for both sides to trade accusations.
Extremist Islamic rebels overran a village in central Syria populated by Assad's Alawite minority, killing at least 40 people on Sunday, activists said. Half of the victims in the attack in Maan were civilians and the rest were village fighters defending their homes, said the British-based Syrian Observatory for Human Rights.
Syrian state media described the attack as a "massacre" perpetrated by terrorists. Extremist Sunni Islamic fighters have played an increasingly prominent role among the rebel fighters, dampening the West's support for the rebellion to overthrow Assad.
The raid on Maan is likely to bolster efforts by the Syria government delegation to claim in Geneva that the three-year uprising against Assad is dominated by al Qaida extremists.
Asked about the Maan killings, Mr Safi said the regime wants to shift the blame on others.
"Yes there are violations by some gangs ... because of the political and security chaos but the primary responsibility lies on the regime," he said. He accused the government of carrying out "war crimes" by bombing civilians with makeshift barrel bombs - crude weapons packed with explosives, fuel and metal - that have wreaked havoc in opposition-held areas, particularly in the northern province of Aleppo and in the Damascus suburb of Daraya.
In the first round of peace talks, Mr Brahimi had pressed for an agreement on aid deliveries, hoping such confidence-building measures would build momentum. But even such small steps were not achieved.
A deal was clinched last week for a three-day truce in rebel-held parts of Homs to secure the evacuation of hundreds of trapped civilians and the entry of humanitarian aid convoys. That effort was disrupted on Saturday as trucks carrying supplies into Homs came under heavy fire. Both sides traded accusations over who was responsible.
The aid effort resumed on Sunday, with more than 600 people evacuated from Homs.
Violence this week has also disrupted UN food parcels destined for the besieged Palestinian refugee camp of Yarmouk near Damascus.
More than 130,000 people have been killed in the war, activists say, and millions have been forced to flee their homes.
Meanwhile, a watchdog agency says Syria has shipped a third batch of chemical weapons material out of its territory.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said in a statement the Syrian material has been transferred to a Norwegian cargo vessel. It said that the ship is being accompanied by a naval escort including ships from China, Denmark, Norway and Russia, and will be joined by Britain when it reaches international waters.
The Hague-based OPCW, which is overseeing Syria's attempts to destroy its chemical weapons stockpile, said that an unspecified amount of chemicals used in making weapons has also been destroyed within Syria.
Syria has missed several deadlines on the timetable to have its chemical weapons eradicated by June 30, but insists it will meet the final deadline.