Veteran envoy Staffan de Mistura said he plans to invite both sides back to Geneva later this month for a new round of talks, which will include the issue of counter-terrorism at the request of Damascus.
"The train is ready, it is in the station, it is warming up the engine. Everything is ready, it just needs an accelerator," he said at the end of nine days of talks in the Swiss city.
"I believe that we have a clear agenda now in front of us," he told reporters.
The Geneva negotiations, the first since last April, aimed at ending a conflict that began in March 2011 with protests against Syrian President Bashar al-Assad. Its seventh year begins on March 15.
Since then more than 310,000 people have been killed and hundreds of thousands have fled the country, fuelling instability in neighbouring countries and creating Europe's biggest migrant crisis since World War II.
The warring Syrian sides have been joined in Geneva by envoys of key parties including notably Russia, a major ally of Damascus.
READ THESE STORIES TOO
Syrian government, Russia drive IS out of Palmyra again
Counter-terrorism
But as in previous talks the focus was almost exclusively on the agenda.
Under UN Security Council Resolution 2254 they should be framed in three "baskets" or areas of discussion: governance, constitution and elections.
But Damascus pushed hard for counter-terrorism to be added to the agenda, against fierce resistance from the opposition who said the Syrian regime was "stalling" the talks to avoid engaging with political transition.
The main opposition High Negotiating Committee (HNC) said the latest Geneva talks were "more positive" than previous rounds.
"We are closing this round without (a) clear result... but I can say this time was more positive," HNC delegation chief Nasr al-Hariri told reporters.
"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 de Mistura.
The antagonism has been clear in briefings after each session of talks with de Mistura, with the Syrian regime delegation chief Bashar al-Jaafari lashing out at "terrorists" in the HNC.
The HNC meanwhile lamented the lack of a genuine "partner for peace".
The talks had hardly begun last weekend when they were rocked by a suicide assault which killed dozens in Syria's third city Homs.
Al-Jaafari demanded after the attacks that terrorism be made a "priority" in Geneva.
Speaking at the end of the talks Friday, the UN mediator conceded that for the moment face-to-face talks are unlikely.
But he held up a photograph of the opening ceremony of the talks last Thursday, when both sides gathered in the same room, albeit only to hear a welcome address by de Mistura.
"This picture is much more than iconic. It is highly symbolic. This was a very special moment," he said, adding: "A psychological barrier was broken".
And he added: "I know there are still people in Syria who still believe that there is a military option or a military solution.
"That is fantasy," he said, adding that only a "political solution that addresses the legitimate aspirations of the Syrian people" would ultimately prevail.
