Lakhdar Brahimi acknowledged the agreement on the city of Homs to be implemented as soon as Monday fell short of his hope to send a humanitarian aid convoy. But, he said, "to bring Syria out of the ditch in which it has fallen will take time."
Homs, one of the first cities to rise against President Bashar Assad, once more came under mortar attacks from the government.
Brahimi defended the pace of the talks, which have yet to touch upon the issue of Assad's future.
"I think being too slow is a better way than going too fast," he said. "If you run, you may gain one hour and lose one week."
Brahimi said the thorniest topic - a possible transitional government - will not come up until at least Monday.
"I think this belittles the importance of this conference and the goal that was drawn for it," said Bouthaina Shaaban, an adviser to Assad.
The Western-backed opposition, made up largely of exiled Syrians, says Assad has lost legitimacy and can no longer lead a country after unleashing the military on largely peaceful protests nearly three years ago.
The government says the rebellion is rife with terrorists and that Assad is the only person able to end the fighting that has killed more than 130,000 people.
In the meantime, the homegrown rebellion has become a regional proxy war between Iran and Saudi Arabia, with foreign fighters flooding in on both sides.
Homs was considered a promising place to start the negotiations.
The city was one of the first areas that plunged into armed conflict in 2011. Neighborhoods in the old city have been ravaged following repeated government assaults to reclaim control from rebels.
The city had a pre-war population of 1 million, but most residents have since fled. Activists say about 800 families are trapped, without regular access to food, medicine and basic necessities.
"The regime is blocking all convoys to Homs and has been doing so for months," said a senior U.S. official, speaking on condition of anonymity because the talks remain sensitive. "The situation in Homs is extremely urgent. Anything the government says to the contrary is false."
Monzer Akbik, an opposition spokesman, said the coalition was still determined to stay for the political talks set to begin Monday. He accused the government of using "stalling techniques."
Syrian activists, including the Britain-based Syrian Observatory for Human Rights, said some rebel-held districts in Homs came under attack Sunday morning by mortars fired by Syrian government forces.
Heavy fighting also continued Sunday in the Qadam neighborhood on the southern fringe of Damascus, where at least 35 rebels and government troops were killed the previous day, said Rami Abdurrahman, the Observatory's director.