Staffan de Mistura said regime and opposition negotiators, gathered for a fourth round of UN-sponsored talks, had a duty to lead Syria out of its bloody "nightmare".
"This is ... Our solemn responsibility ... A historical responsibility not to condemn the future generations of Syrian children to long years of bitter and bloody conflict," he said.
"The Syrian people desperately want an end to this conflict and you all know it .. They are awaiting for a relief from ... Suffering and dream of a new road out of this nightmare," he added.
Persistent violence and deadlock over Syria's political future are clouding the start of "Geneva 4," the fourth round of UN-sponsored talks on the conflict which has killed more than 310,000 people since 2011.
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On the eve of the talks, Russia called on Syrian President Bashar al-Assad to stop bombing during the discussions.
But de Mistura, who had already warned he is not expecting a breakthrough at he talks, warned again Thursday of the challenge facing them.
The ground -- both in territory and diplomatically -- has shifted since the last UN-sponsored talks broke up in April 2016, and the rebels are in a significantly weaker position.
The army has recaptured the rebel bastion of eastern Aleppo and the United States, once staunchly opposed to Assad, has said it is reassessing every aspect of its Syria policy under President Donald Trump.
The latest truce was brokered in late December by opposition supporter Turkey and regime-backer Russia ahead of separate negotiations in Kazakhstan that also involved Iran.
A bitter dispute over Assad's fate also continues to divide the camps.
The HNC has insisted he must leave office as part of any deal, while Damascus has said the president's future is not open for negotiation.