After the final session of the talks, the head of the regime and opposition delegations also insulted each other by calling the other side "terrorists".
Staffan de Mistura on March 31 said after wrapping up nine days of talks in Geneva that the sides appeared "keen" on holding a new round of talks, but he said no date had been set.
At the same time, he underscored: "There are real challenges on the ground, we are seeing them in front of our eyes, but let´s be frank: in spite of them, no one threatened to walk out and every one was serious and business-like in pursuing their points of view".
De Mistura ended rumors about his imminent resignation as special envoy by saying they should be taken seriously only if either he or the United Nations secretary-general announces that he is leaving.
In the talks, de Mistura said, "We were mostly talking about substance".
The discussion had two aspects: political and humanitarian.
"We are in a moment of pushing ahead an agenda".
According to the politician, the delegates are expected to hold talks with Russian Deputy Foreign Minister Mikhail Bogdanov and representatives from Russia's political parties.
Opposition negotiator Nasr al-Hariri said the "terrorist regime" of President Bashar al-Assad had refused to discuss political transition during the talks and said Assad was a war criminal who must step down in the name of peace.
"During this round we presented a number of documents to the special envoy".
He said that there should be no expectations of the talks collapsing or of making a breakthrough, as what matters now is that progress is being made on the agenda, adding that what is important is preparing for the issues that need preparation before beginning actual peace talks, stressing that progress was made in this regard.
Hariri said the United States remained a friend and there had been no "dramatic change", with the US priority being to fight terrorism and limit also the influence of Iran.
The Syrian conflict is thought to have killed over 310,000 people since it erupted in March 2011, with millions more displaced by intense fighting and reigning insecurity. The Syrian Center for Policy Research puts the death toll at more than 470,000.