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 said on Wednesday that terrorism could not be added to the faltering talks.
Government representatives, meanwhile, have accused the opposition of taking the talks "hostage", accusing the rebels of including members of "armed terrorist groups".
Speaking to reporters United Nations diplomat Staffan De Mistura called the round of talks substantive and said the parties had agreed to hold talks in March to discuss to settle the conflict through political means.
De Mistura said the strategy for fighting terrorism - specifically "counter-terrorism, security governance and also medium-term confidence-building measures" - would be on the cards in future discussions.
"We must not permit the Riyadh platform to hold Geneva talks hostage", lead government negotiator Bashar al-Jaafari told reporters, referring to the Saudi-backed opposition, after meeting with UN Syria envoy Staffan de Mistura.
De Mistura said the Geneva talks had the support of key regional player Turkey, which supports the opposition, as well as the allies of Damascus, Russia and Iran.
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The fourth round of talks on the Syrian conflict in Geneva ended with the modest hope that the two sides to the conflict realise that dialogue is the only viable option left to settle the six-year old conflict, which left almost half-a-million people dead, hundreds of thousands injured and no less than 11 million Syrians either displaced or refugees.
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.
Despite periodic displays of division, Western diplomats and analysts who had come to shepherd the opposition through the tricky process credited the motley crew of political dissidents and military commanders for holding their ground.
Since the last round was held a year ago, rebel factions have lost eastern Aleppo, one of their main strongholds in the country, and are increasingly on the back foot as Damascus seeks to consolidate its military gains on the ground.
The UN official also said that the Astana and Geneva efforts complement each other.
"We accepted to enter an open-ended political process", al-Rais said.