"I think it is better that every side goes back and reflects on their responsibility, (and on whether) they want this process to continue or not," Lakhdar Brahimi told reporters in Geneva.
Speaking on the final day of a second round of talks that have been mired from the start by blame-trading over the violence ravaging Syria, he apologised to the Syrian people for not making progress.
"I'm very, very sorry," he said.
Brahimi said that the two sides now at least had reached agreement on an agenda for future talks- if they happen- something they had failed to do throughout the past week.
"At least we have agreed on an agenda. But we also have to agree on how we tackle that agenda," Brahimi said, adding: "I very much hope there we will be a third round."
The opposition has insisted that the focus of the talks must be on creating a transitional government, without President Bashar al-Assad.
The regime representatives have meanwhile stubbornly insist President Bashar al-Assad's position is non-negotiable and refused to discuss anything beyond the "terrorism" it blames on its opponents and their foreign backers.
Brahimi said today that if the sides returned for a third round they would discuss violence and terrorism first, then the transitional governing body (TGB), followed by national institutions and finally national reconciliation and national debate.
However, he said the regime side had baulked at his suggestion that they spend one day on violence and the next on political transition "which raises the suspicion of the opposition that the government doesn't want to discuss TGB at all."
The veteran peacemaker said a break in the talks was needed.
"I hope that this time of reflection will lead the government side in particular to reassure the other side" that the issue of political transition will be discussed seriously, Brahimi said.
The UN mediator said he himself would soon travel to New York to report to Secretary General Ban Ki-moon about the situation, while he expected the parties to consult with their leaders.
"We will hear from one another in the not-too-distant future," he said.