BEIRUT — Syrian activists and officials said mortars were fired early on Saturday in the besieged Old Homs district in violation of a ceasefire that is intended to allow the evacuation of civilians and delivery of aid to people trapped in central neighborhoods.

The opposition Shaam News Network said the mortar fire had coincided with the entry of two U.N. cars, accompanied by rebel forces, into the besieged districts. It said some of the mortar bombs targeted an area on the edge of Homs near to where negotiations over the aid operation were taking place.

It blamed Assad's forces for the barrage.

It was unclear whether the cars were turned back and it was not immediately possible to verify the reports. Authorities restrict media operations in Syria and the country's warring parties often accuse each other of attacking civilian targets.

State news agency SANA quoted Homs governor Talal al-Barazi as saying "armed terrorist groups broke the truce this morning in the Old City of Homs, firing mortars at the police building".

Syrian authorities describe all armed opposition against Assad as terrorism.

The Syrian Observatory for Human Rights, an anti-Assad monitoring group, quoted activists in Homs as blaming Assad's forces for the bombardment. It said five explosions shook the area Saturday morning.

A United Nations convoy with food and medical supplies was on standby on Saturday to enter the Old City and deliver its first shipment of aid to the district since mid-2012.

On Friday 83 civilians were evacuated from central Homs. Aid workers said 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.