A new round of Astana talks is due on March 14, and Russian officials have said the Geneva negotiations could resume on March 20.
"The train is ready, it's in the station, it's warming up its engine", he said.
While direct talks between Syrian government and opposition delegations have yet to happen, de Mistura reminded that this remains an objective for discussions seeking to broker a political end to the six-year conflict.
The opposition has been hesitant to add "terrorism" to the agenda over fears that the government would use it to sideline discussions on political transition - the opposition's central goal.
"It was first time we discussed in an acceptable depth the issues of the future of Syria and political transition", he added after the talks, the fourth mediated by United Nations envoy Staffan de Mistura.
Syria's first U.N. -led peace talks in nearly a year ended on Friday without breakthrough but the United Nations mediator said the warring parties now had a clear agenda to pursue a political solution to the country's six-year-long conflict.More news: Five trends at Mobile World Congress
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Government representatives, meanwhile, have accused the opposition of taking the talks "hostage", accusing the rebels of including members of "armed terrorist groups".
The main Syrian opposition at the talks is the High Negotiations Committee (HNC) but there are also two smaller dissident groups which have no military muscle but enjoy Moscow's blessing as opposition voices.
He said all parties understood they were working toward implementation of a UN Security Council framework that sets out a "clear timetable" to draft a new constitution for Syria within 12 months and for free and fair elections to be held within 18 months.
Damascus sought a unified Syrian opposition. Western diplomats said they understand that the United Nations represents their best shot at a political transition, however slowly the process proceeds.
"I really want to express my gratitude to the Syrian invitees for engaging with me and my colleagues in serious discussions during these last nine days", the special envoy told press.
The HNC said the priority should be a political transition that excludes sitting president Bashar Al-Assad-a request that other opposition groups call unrealistic.