An aid convoy has come under attack in the besieged Syrian city of Homs, threatening a UN-led operation to bring food and medicine to 2,500 people and evacuate civilians trapped by fighting between rebels and government forces.
Saturday's violence threatens to unravel a humanitarian deal for Homs which was the first concrete result of talks launched two weeks ago in the Swiss city of Geneva to try to end Syria's civil war.
The Syrian Arab Red Crescent said mortar bombs landed close to its convoy and shots were fired at its lorries, wounding one of its drivers.
In-depth coverage of escalating violence across Syria
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.
The aid had been held up for months in a UN warehouse in a nearby government-controlled area.
The UN says it wants to distribute food for 2,500 people, along with medical kits, bedding, cash and other support for those leaving or those choosing to stay in the Old City.
Talal al-Barazi, the governor of Homs, told the state news agency SANA that aid was distributed to two neighbourhoods, Bustan al-Diwan and Hamidiyeh.
Video footage published by activists showed eight white cars with UN markings and a lorry stopped at a narrow street corner strewn with rubble.
A man with a blood on his face was led into a nearby building, before a blast struck on the far side of the vehicle. It was not clear how much damage it caused.
The UK-based Syrian Observatory for Human Rights said five people, including two rebel fighters, were killed and 20 wounded by mortar bombs in the Old City of Homs.
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.
|Turkish Prime Minister Recep Tayyip Erdogan discusses the Geneva 2 talks in interview to Al Jazeera
Barazi said two aid vehicles entered the Old City but "terrorist groups prevented the entrance of other vehicles by firing mortar rounds on the road".
"Terrorists broke the truce this morning in the Old City ... by launching mortar rounds at the police headquarters in the Saa area," he said.
Opposition activists from Homs' Unified Media Office said Assad's government was responsible for the attacks.
Al Jazeera's Nisreen El-Shamayleh, reporting from Beirut in neighbouring Lebanon, said the ceasefire, which began on Friday, may be extended to compensate for the volatile situation.
The conflict in Syria has killed 130,000 people, driven millions from their homes and devastated cities - particularly Homs, a centre of protest when the 2011 uprising against 40 years of Assad family rule first erupted.
At the Geneva peace talks, which resume on Monday, Lakhdar Brahimi, the international mediator, has been pushing for agreement on aid deliveries and prisoner releases.
Even the humanitarian talks have taken time and delivered only modest achievements, the first of which was the evacuation on Friday of 83 women, children and elderly men from Homs's Old City.
Aid workers said many showed signs of malnutrition.
In fighting in Aleppo on Saturday, 20 people were killed by barrel bombs dropped by Syrian army helicopters, the Syrian Observatory said.
Barrel bombs were also dropped on Daraya, a rebel basion southwest of Damascus, while 16 people were killed in unrest in the southern province of Deraa, the Syrian Observatory said.
Heavy fighting was also reported in the eastern province of Deir Az zor after the al Qaeda-linked Jabhat al-Nusra and another group, Ahrar al-Sham, attacked the Islamic State of Iraq and the Levant (ISIL). At least 20 people were killed, according to the Syrian observatory.