The talks, which yielded no apparent breakthrough on Monday's first day, could have been the first face-to-face negotiations between the regime and the armed opposition since Syria's conflict erupted in 2011, but the rebels backed out.
"We are not far from a final declaration," UN envoy Staffan de Mistura said.
"There are very intense discussions because this is not about a paper, this is about a cessation of hostilities which means Syrian lives."
Rebel spokesman Yehya al-Aridi said the rebels would not sign a final declaration coming out of the talks, saying it would be issued by their sponsors, rebel backer Turkey, regime ally Russia and possibly Iran.
He added the final declaration was a "general statement" that is "not meant to be signed by the parties".
The rebels rejected face-to-face talks because of the regime's continued bombardment and attacks on a flashpoint outside the capital Damascus.
The two sides sat at the same table for the opening statements, but spent the rest of Monday negotiating via mediators.
Different objectives
The rebels have insisted the talks focus on bolstering a frail truce brokered by Turkey and Russia last month, while the regime has called for a political solution to the nearly six-year conflict and for rebels to lay down their arms in exchange for an amnesty deal.
Rebel spokesman Osama Abu Zeid said ceasefire violations and threats of forced displacements were hindering the negotiations, and that the rebels would focus on the truce in Tuesday's talks.
In addition to having different objectives, the two sides also disagree about the role of the talks' three organisers, Russia, Turkey and Iran.
A member of the rebel delegation told AFP on Monday that the group would agree to have Russia serve as a guarantor of the current ceasefire but not Iran, another backer of President Bashar al-Assad.
The Syrian regime, meanwhile, has said it would refuse to hold government-level talks with Turkey and sign any document bearing the signature of a Turkish official -- suggesting this would include any deal to come out of the talks.
The latest diplomatic initiative to end the bloodshed in Syria comes one month after regime forces, aided by Russia and Iran, dealt a crushing blow to the rebels by retaking full control of the country's second city Aleppo.
More than 310,000 people have been killed and more than half of the country's population displaced since Syria's conflict erupted in 2011 with protests against Assad's rule.
