The truce, which began Friday, has been shaken by continued shelling and shooting that prevented some residents from escaping and limited the amount of food aid officials have been able to deliver into the besieged neighborhoods.
UN humanitarian chief Valerie Amos sharply criticized the two sides, saying UN and Syrian Red Crescent workers were "deliberately targeted."
The drama in Homs, where Amos said around 800 civilians have been evacuated so far, played out as activists yesterday reported new sectarian killings in Syria's civil war.
The violence further rattled peace talks that entered their second round yesterday in Geneva and which quickly became mired in recriminations between President Bashar Assad's government and the opposition in exile.
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The two sides' first face-to-face meetings adjourned 10 days ago, having achieved little. This time, the two appeared even further apart, with no immediate plans to even sit at the same table. UN-Arab League envoy Lakhdar Brahimi was holding separate talks with each side.
But Syrian Deputy Foreign Minister Faisal Mekdad said the issue of Assad stepping down was not on the agenda. "Please tell those who dream of wasting our time here in such a discussion to stop it," he told a reporter.
The events of the past few days have only underscored each side's position. The government says it is trying to defeat an extremist, al-Qaida-style insurgency. Syria's opposition, in turn, points to government blockades of dozens of rebel-held areas that have caused widespread hunger and sickness among civilians as proof of the cruelty of Assad's rule.