The UN's humanitarian chief Valerie Amos has vowed to push on with relief deliveries to civilians trapped in Homs, after a Red Crescent aid convoy was attacked.
Amos' comments come after the convoy came under mortar and gun attack on Saturday in the Syrian city, despite an agreed three-day ceasefire which began on Friday.
"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 released late on Saturday.
"Today's events serve as a stark reminder of the dangers that civilians and aid workers face every day across Syria," she added.
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Saturday's attack threatened a UN-led operation to bring food and medicine to 2,500 people in Homs Old City and evacuate civilians trapped by months of fighting.
The Syrian Arab Red Crescent said mortar fire landed close to its convoy and shots were fired at its lorries, 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," Amos 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."
At least nine Red Crescent and UN vehicles were trapped in Homs for several hours after dark when the explosions struck, but the team managed to leave shortly before 10pm local time (20:00 GMT), leaving two damaged lorries.
"Although the team was shelled and fired upon, we managed to deliver 250 food parcels [and] 190 hygiene kits and chronic disease medicines," the Red Crescent said.
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The aid had been held up for months in a UN warehouse in a nearby government-controlled area.
Syrian authorities blamed the attacks on rebels but opposition activists held President Bashar al-Assad's forces responsible.
The UK-based Syrian Observatory for Human Rights said five people, including two rebel fighters, were killed and 20 wounded by mortar bomb attacks on the Old City area.
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.
Sunday is due to be the final day of a three-day ceasefire which Russia, a close supporter of President Bashar al-Assad, said had been agreed to allow the aid to be brought in and civilians moved out.