A convoy delivering aid to civilians trapped in Syria's Homs city on Saturday was fired on, wounding at least one driver, as the regime and rebels accused each other of breaking a humanitarian truce.
The violence comes a day after 83 children, women and elderly people who survived more than 600 days of a choking army siege were evacuated in a UN-supervised operation.
Clashes erupted in the morning in rebel-held areas of Homs' Old City, delaying the delivery of aid.
Hours later a Syrian Red Crescent convoy came under attack.
"Shots fired targeting aid trucks and the team," the Red Crescent said on Twitter.
"Mortar shells falling in close proximity near the team and aid trucks that moved into Old City," it added, saying a driver was wounded.
State news agency SANA said four Red Crescent volunteers were wounded "by gunshots fired by armed terrorist groups," as an aid convoy was entering the Old City.
And Homs governor Talal al-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."
There was no immediate claim of responsibility for the attack, and it was unclear if the violence will stop a hard-won deal for the United Nations to deliver much-needed aid to civilians in Homs.
The regime calls rebels fighting to topple the government "terrorists".
The evacuation and aid delivery were made possible by a surprise UN-brokered deal between the government and rebels to observe a three-day "humanitarian pause" in hostilities.
But five explosions were heard at 8:30 am (0530 GMT) in the besieged neighbourhoods, the Britain-based Observatory said.
The regime and opposition blamed each other for the clashes.
"The armed terrorist groups broke the truce this morning in the Old City of Homs by launching mortar rounds at the police headquarters in the Saa area," said Barazi.
Activists said the regime was responsible.
"The besieged areas have been pounded with mortar rounds since Saturday morning," said the Unified Media Office activist group in the besieged areas.
"The shelling is also targeting the road on which the humanitarian aid is supposed to be transported."
The exiled opposition expressed concern the aid delivery could be aborted, saying it would be "devastating" for besieged civilians.
Desperately needed food and medicines have been held up for months in a UN warehouse in a government-controlled area.
"Aid convoy is now being loaded and prepared to go to the Old City of Homs," the Red Crescent tweeted just before 11:00 am (0900 GMT).
A government source later said "the truck carrying relief has been waiting from 10:00 am (0800 GMT) to enter the city to now (1430 GMT)."
The UN says trucks would carry emergency rations for 2,500 people, medical kits and bedding, as well as cash and other support for the "immediate needs both of those who choose to be evacuated from the area and of those who remain inside" after Friday's evacuations.
Activists say trapped civilians have been surviving for months on little but olives and wild cereals.
UN humanitarian chief Valerie Amos hailed the truce and said there were plans for more evacuations and aid deliveries in the next few days.
A cleric in the rebel enclave, Abdul Hareth al-Khalidi, told AFP a second operation to evacuate civilians would take place on Sunday.
Homs, much of which has been reduced to rubble, was dubbed "the capital of the revolution" by activists before a bloody 2012 counter-offensive by regime forces recaptured much of the city.
The army blockaded the remaining rebel-held areas after their 2012 assault. They tightened the noose last summer by capturing the town of Qusayr, which cut off rebel supply lines to neighbouring Lebanon.
Meanwhile, new attacks with barrel bombs dropped from helicopters killed at least 20 people in the northern city of Aleppo, in a campaign that has left hundreds dead since December, the Syrian Observatory for Human Rights said.
Similar attacks were also reported on Daraya in the south.
The raids came as Al-Qaeda affiliate Al-Nusra Front and allied Islamist rebel groups launched a new offensive against the jihadist Islamic State of Iraq and the Levant in Syria's oil-rich east.
More than 136,000 people have died in the nearly three-year civil war.