U.N. humanitarian chief Valerie Amos expressed deep disappointment on Saturday after an aid convoy came under fire in a besieged rebel district of the Syrian city Homs, though she vowed to keep pushing to deliver humanitarian assistance to Syria's neediest.

"I am deeply disappointed that the three-day humanitarian pause agreed between the parties to the conflict was broken today and aid workers deliberately targeted," Amos said in a statement.

"Today's events serve as a stark reminder of the dangers that civilians and aid workers face every day across Syria," she said.

Saturday's attack threatened a United Nations-led operation to bring food and medicine to 2,500 people in Homs and evacuate civilians trapped by months of fighting in the Syrian city.

The Syrian Arab Red Crescent (SARC) said mortar fire landed close to its convoy and shots were fired at its trucks, wounding one of its drivers.

"I continue to call on those engaged in this brutal conflict to respect the humanitarian pause, ensure the protection of civilians and facilitate the safe delivery of aid," she said. "The United Nations and our humanitarian partners will not be deterred from doing the best we can to bring aid to those needing our help."

Syrian authorities blamed the attacks on rebels but opposition activists said President Bashar al-Assad's forces were responsible for them, as well as earlier mortar fire that delayed the start of the operation on Saturday morning.

The humanitarian deal for Homs was the first concrete result of talks launched two weeks ago in Geneva to try to end the country's nearly three-year-old civil war that has killed over 136,000 people.

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