Some rebels and activists in Homs have been negotiating a truce for at least two months, but the bulk of the rebels refused to agree until the final, violent push of fighting, activists said.
U.N. deputy spokesman Farhan Haq said Secretary-General Ban Ki-moon has been pushing for an agreement. The world body also helped oversee the evacuation of hundreds of civilians from Homs' rebel-held neighborhoods in February.
Activists said the 48-hour truce began Friday, and that some 1,000 rebels belonging to all factions, including the al-Qaida linked Nusra Front, were expected to begin evacuating on Saturday to rebel-held provincial towns north of Homs.
Still, the deal could collapse if there are last-minute disputes over the terms of evacuation and some rebels decide to hold out.
A Syrian opposition figure said some sticking points remained, including whether the rebel-held Waer district just outside the Old City would be included in the deal.
Speaking on condition of anonymity because negotiations were ongoing, he said the government was also demanding that rebels hand over military maps of tunnels and explosives.
In Homs on Friday, Syrian soldiers with machine guns at checkpoints appeared relaxed. The facades of nearby buildings were battered by shrapnel from mortar fire in recent days.
At the checkpoints, posters of smiling Assad hung off sandbags and makeshift barriers made of truck tires, with the Syrian national flag fluttering overhead.
The skyline of the battle-shattered Waer district resembled a heap of ruins and twisted metal. Several tall buildings are now skeleton structures, destroyed by government heavy artillery a few months ago after rebels reportedly took up positions there and fired on an oil refinery on the outskirts of the city.
Tilawi and other activists cautioned that the deal was extremely fragile.
"We don't trust the regime. If there's any shooting or traps set, the whole thing will fall apart," he said.
Also Friday, two car bombs struck two small pro-government villages in the central Syrian province of Hama, killing 18 people, including 11 children, state-run television said. The villages, Jadreen and Humayri, are about a 20-minute drive apart, and it wasn't immediately clear if the two attacks were coordinated.
More than 150,000 people have been killed in Syria's conflict since it began in March 2011.
Karam reported from Beirut. Associated Press writers Diaa Hadid in Beirut and Edith M. Lederer at the United Nations contributed to this report.
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