Forces loyal to Syrian President Bashar al-Assad shelled the battered city of Homs on Saturday, according to opposition activists.
It would be the first bombing since a ceasefire came into force two days ago.
Several people are said to have been wounded.
On Friday, Syrians took part in rallies, hoping the truce would protect them.
But there are reports that five demonstrators were shot dead.
Syria’s state news agency SANA said “armed terrorists” killed six people, including an army major.
The UN Security Council is tentatively scheduled to vote today on sending in monitors to oversee the ceasefire.
“At the moment we have the advance team standing by to board planes and to get themselves on the ground as soon as possible,” said Ahmad Fawzi, spokesman for joint UN-Arab League special envoy Kofi Annan.
It remains unclear whether Russia – one of Assad’s allies – can be persuaded to support the draft resolution, which calls on Syria to allow in a team of up to 30 unarmed military observers.