Brahimi had held final discussions with both sides in a last-ditch bid to break the stalemate between the Syrian government and opposition.
Brahimi said the last session of the second round of talks in Geneva was "as laborious as all the meetings we have had, but we agreed on an agenda for the next round when it does take place".
He said the points to be discussed included violence and terrorism, the transitional governing body, national institutions and national reconciliation.
However, the mediator said the Syrian government wanted to first deal with the issue of combating terrorism and had refused to deal with any other points until that was resolved.
"I apologise that these two rounds have not come out with very much," he said.
The three-year-old Syrian conflict has seen the deaths of more than 130,000 people and is destabilising the country's neighbours. The mainly Sunni Muslim rebels have drawn support from radical Sunni groups such as al-Qaeda and foreign militants.
Shi'ite countries and militias have thrown their weight behind president Bashar al-Assad, who is from Syria's minority Alawite sect, an offshoot of Shi'ite Islam.
Brahimi said both sides at the peace talks needed to reflect on their responsibilities. He said he hoped this would "lead the government side in particular to reassure us that when they speak of implementing the Geneva Communique they do mean a transitional governing body, exercising full executive power, will be the main objective".
Meanwhile, Peter Maurer, head of the International Committee of the Red Cross, said yesterday that Syria's government and opposition are still not honouring basic tenets of international humanitarian law despite the evacuation of the Old City of Homs. About 1000 people are believed to have left the city, mostly women, children and the elderly, though many more remain trapped.
The deal to get civilians out of the Old City was initially seen as an ice-breaker that would kickstart peace talks in Geneva. Maurer said the evacuation did not herald any wider improvement in humanitarian access in Syria, where the United Nations says it cannot access up to three million people in need.
"Negotiations with the Syrian authorities and opposition groups have not resulted in meaningful access or a firm commitment to respect the basic principles of international humanitarian law," said Maurer. "This pattern has played out in Homs over the last week."
The basic tenets of the law were simple, he said. The parties to the conflict had to provide for the basic needs of the population or authorise impartial humanitarian action and, if necessary, assisting people wanting to leave.
Anyone choosing to stay behind remained protected by international humanitarian law and must not be attacked, he said.
"If humanitarian work is to be meaningful and effective, it must be supported by the parties. This means more than one-off distributions and operations: it requires repeated access to the areas affected by the fighting."