UN humanitarian chief Valerie Amos has insisted that the UN and aid agencies will not be deterred by Saturday's attack on a convoy bringing supplies into the Syrian city of Homs.
Baroness Amos said she was sad that a three-day ceasefire to allow aid into the old part of Homs had been broken.
The events were "a stark reminder of the dangers that civilians and aid workers face every day across Syria".
She said the UN would do "the best we can" but needed "safety guarantees".
The convoy came under attack from mortars and gunfire as it was leaving Homs on Saturday, the second day of the humanitarian ceasefire.
Syrian authorities have blamed the attack on rebels, but they in turn say that President Bashar al-Assad's forces were responsible for the incident.
The Red Crescent, in a joint operation with the UN, is trying to deliver food, water and medicine by truck to some 3,000 civilians in rebel-held areas.
After an initial delay due to mortar fire, a team set off on Saturday to carry out their mission.
On their way out of the city, they came under attack. The Red Crescent said mortars landed close to their convoy and shots were fired at their trucks. A driver was reportedly injured.
Red Crescent spokesman Khaled Erksoussi said the group took refuge in "buildings and safe areas" until they were able to get out, shortly before 22:00 local time (20:00 GMT).
They had to leave two of their damaged trucks behind.
"Although the team was shelled and fired upon, we managed to deliver 250 food parcels, 190 hygiene kits and chronic diseases medicines," the Red Crescent said in a statement.
An unverifiable video, promoted by activists, shows UN vehicles trapped and under bombardment in the dark, as a wounded man is stretchered away.
Both sides blame each other for the renewed violence, but the BBC correspondent in Lebanon, Jim Muir, says that unless the videos have somehow been faked, the UN and other vehicles appear to be inside the rebel-held quarter, and being fired on from outside.
Baroness Amos said on her Twitter feed that she was "deeply disappointed [the] humanitarian pause was broken today in Old Homs".
She said that the aid workers had been deliberately targeted and she commended their courage.
The BBC's Jim Muir: "Relief teams trying to get help to trapped civilians found themselves sheltering for their lives"
A group of civilians is due to be evacuated from the city on Sunday - part of an ongoing attempt to make sure civilians who want to leave the area can do so in safety.
But out correspondent says the breakdown in the ceasefire clearly casts doubt on the future of the agreement.
A Red Crescent spokesman told the BBC that guarantees were needed for the humanitarian operation to continue.
Homs has been a key battleground in the uprising against President Assad.
The army launched a series of big attacks to recapture rebel areas in the Old City in the beginning of 2012, with almost daily bombardments.
Thousands have been killed, large areas have been reduced to rubble and many neighbourhoods lie in ruins.
Another round of talks is scheduled to begin on Monday and the Syrian government has confirmed it will attend.