Only three convoys have reached rebel-held towns over the past two months in what UN aid chief Stephen O'Brien described as "a zero or near-zero" rate of assistance to Syrians living under siege in the nearly six-year war.
This week, two convoys were scheduled to reach opposition-held Waer near the central city of Homs, but one was forced to turn back due to sniper fire en route on Sunday.
"The drivers and trucks were temporarily detained, and some drivers were reportedly roughed up, but have since been released, without humanitarian supplies, and everyone is safe and accounted for," O'Brien said yesterday.
The United Nations last week had urged the Syrian government to allow the aid to reach Waer as a "goodwill gesture" ahead of peace talks opening on Thursday in Geneva.
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O'Brien condemned the incident as a "blatant disregard for the protection of humanitarian workers" and said efforts would continue to try to reach Waer, where 50,000 civilians have not received any assistance in nearly four months.
Rebel-held Madaya and government-controlled Kefraya are covered by a complicated arrangement to provide tit-for-tat aid, but the deal has gone awry.
The war in Syria, which will enter its seventh year next month, has killed 310,000 people while more than half of the country's population -- or nearly 13 million people -- have been driven from their homes.
UN envoy Staffan De Mistura said he is not expecting a breakthrough in the peace talks, the first between the Syrian government and the weakened opposition since April last year.