United Nations-led efforts to deliver humanitarian aid to a rebel-held, besieged district of central Homs have been hampered after an aid convoy came under fire and parts of the devastated city were bombarded by mortar fire.
The Syrian Red Crescent tweeted that one of its drivers had been injured after its cars were shot at, forcing them to turn back. Syrian state television later said that four aid workers were wounded in the attack.
The aid agency did not hold anyone responsible for the incident while state television blamed rebels.
There were conflicting reports about whether any aid had reached people in the rebel-held Old City, the most in-need area, after the violence erupted and violated a humanitarian ceasefire.
Al Jazeera's Nisreen El-Shamayleh, reporting from neighbouring Lebanon, said further attempts to deliver food and medical supplies were not expected on Saturday and that the ceasefire, which began on Friday, may be extended to compensate for the volatile situation.
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The UK-based Syrian Observatory for Human Rights reported that two Homs residents were killed and several more were wounded by mortar fire.
The mortar fire appeared to target an area in Homs near a point where United Nations and Syrian Arab Red Crescent vehicles were preparing to enter the besieged area which has been devastated by Syria's nearly three-year conflict.
The country's bitterly-divided warring sides blamed each other for the shelling, with the state news agency SANA quoting the Homs governor, Talal al-Barazi as saying:
"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.
"Military commanders were told to exercise maximum restraint in order to allow the evacuation of civilians who are being held by the armed groups in the Old City."
Syrian authorities routinely describe all armed opposition groups as "terrorists".
But opposition activists blamed President Bashar al-Assad's forces.
"The besieged areas have been pounded with mortar rounds since Saturday morning. The shelling is also targeting the road on which the humanitarian aid is supposed to be transported," AFP reported them as saying.
On Friday 83 civilians were evacuated from central Homs, with aid workers saying some of them showed signs of malnutrition after living under siege for a year and a half in one of the strongholds of the 2011 uprising against Assad, which became an armed insurgency after his forces cracked down on protests.
The convoy on standby would be the first shipment of UN food and medical supplies to the district since mid-2012.