Activists trapped in the besieged districts of Syria's Homs fear a planned evacuation of civilians may not go ahead Sunday, a day after shelling targeted an aid convoy and killed five residents.

The planned evacuation is part of a UN-supervised deal to allow scores of civilians to leave a handful of Homs neighbourhoods that have been under a tight army siege for more than 600 days.

The deal also includes the distribution of humanitarian aid for some 3,000 people trapped by the siege.

But shelling on Saturday targeting a UN and Red Crescent convoy killed five people and wounded 20 others, according to the Syrian Observatory for Human Rights, a Britain-based group relying on a network of sources inside Syria.

President Bashar al-Assad's regime and the rebels fighting to oust him traded blame for the attack, which prevented the delivery of some of the humanitarian aid.

"We don't know yet whether the evacuation of civilians will continue today. You saw what happened yesterday. The shelling against the Old City (of Homs) was crazy," said Abu Bilal, an activist trapped there since June 2012.

"We hope more aid will come in, and we hope the civilians can be evacuated, but we don't know whether that will happen. We are afraid that we will only see more of yesterday's shelling."

The area was calm Sunday morning, as both sides appeared to be honouring a three-day truce that started Friday.

In a statement late Saturday, Homs governor Talal al-Barazi said the authorities are committed to the humanitarian operation.

"The Syrian Arab army is committed to the truce put in place to allow aid in and to give civilians safe passage out from the Old City despite the (rebel) armed groups' violations," Barazi said in a statement sent to AFP.

Abu Bilal said it would "make no sense" for the rebels to attack the aid convoy, adding that a rebel commander was killed in Saturday's shelling.

Rights groups have regularly condemned the siege of Homs' rebel districts, where at least 1,200 women, children and elderly people are among some 3,000 surviving on little more than wild herbs and olives.

The humanitarian operation under way was kick started by talks in Switzerland last month that brought together regime and opposition representatives.

The peace talks made little visible progress towards reaching a political settlement for the nearly three-year war, which has claimed more than 136,000 lives and displaced millions of people.

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