Talal Barrazi, the governor of Homs province, told the Lebanon-based Al-Mayadeen TV that the attack occurred late in the afternoon and that the trucks were targeted by two roadside bombs and a mortar shell.
He later told Syrian state TV that two trucks were able to reach opposition-held neighborhoods earlier in the day. Al-Mayadeen also reported that two trucks, carrying 250 food parcels, were able to cross into rebel-held areas Saturday.
Syrian TV said four members of the Syrian Arab Red Crescent were wounded by rebel fire in the area, but gave no further details.
Barrazi said about 100 civilians expected to be evacuated from rebel-held areas had yet to arrive. On Friday, 83 children, women and elderly people on wheelchairs were evacuated from Homs, the first people to leave the area in months, the U.N. said.
Syrian forces loyal to President Bashar Assad have prevented the entry of food and medical aid into rebel-held parts of the city for over a year, badly affecting hundreds of civilians holed up in the areas. An agreement had called for a three-day truce to allow the evacuation of some civilians and the entry of food shipments.
Al-Mayadeen aired live footage from the city's Clock Square showing two white trucks identified with Syrian Arab Red Crescent markings as they returned after coming under fire. The station's reporter in the area said the radiator of one of the trucks was hit by a bullet.
"After the vehicles drove about 200 meters, two roadside bombs went off, and when they kept going a mortar round fell in the area coming from the direction of Hamadiyeh," said Barrazi, referring to a rebel-held central neighborhood.
Homs city was one of the first areas to rise up against Assad in 2011 and has been particularly hard hit by the war. Over the past year, the government has regained control over much of the city, except for a few neighborhoods in the historic center.
A coalition of exiled Syrian activists said Saturday they feared the agreement would be used as a "prelude to the regime destroying the city."
"It has used similar deals to buy time to strengthen its positions on the ground and to kill more civilians," the Syrian National Coalition said.
Also Saturday, military aircraft dropped barrels bombs on rebel-held areas in the northern city of Aleppo, killing at least 15. The bombings are part of a weekslong campaign by Assad's forces to wrest control of the city, parts of which were seized by rebels in mid-2012.
Activists say the massive barrel bombs often prepare the way for a government advance. But the crude weapons — cylinders packed with explosives and shrapnel dropped usually from helicopters — cannot be aimed precisely and have killed hundreds of civilians.
More than five such bombs exploded on roads in the adjacent areas of Masaken Hanano and Haidariyeh, said Mohammed Wissam of the Aleppo Media Center. Other bombs fell on the Kalaseh district, the Aleppo Media Center reported. The activist collective, the Local Coordination Committees, also reported the bombings.
Wissam said four people were killed in Masaken Hanano and another 11 were killed in Haidariyeh.
Video footage of one incident started with a thundering explosion followed by a column of smoke billowing to the sky. Men rushed about a damaged building, pulling out smashed bodies. They carried them on makeshift stretchers of cardboard and blankets, laying the dead on a sidewalk. One man placed a severed limb next to a sprawled body. Men rushed to an ambulance carrying a white sheet laden with body parts; they said they had collected four bodies.
The videos could not be independently authenticated but appeared genuine and corresponded with Associated Press reporting of the event.
Thousands of civilians have fled other pummeled rebel-held areas, said activist Wissam. Their numbers add to the millions who have been displaced during Syria's three-year uprising that began as largely peaceful demonstrations in March 2011.