There was shelling of several rebel-held neighbourhoods in the central city of Homs, according to the Local Co-ordination Committees.
The Britain-based Syrian Observatory for Human Rights said “tens of army vehicles” were deploying in the southern town of Maaraba amid intense shooting.
The ceasefire is scheduled to begin at 6am tomorrow.
The new violence came as international envoy Kofi Annan appealed to Syria’s key ally Iran to support his plan to end the violence, saying that “any further militarisation of the conflict would be disastrous”.
Mr Annan spoke to reporters after talks on Tuesday with foreign minister Ali Akbar Salehi.
The UN-Arab League envoy has been pushing Damascus to withdraw its troops from cities and halt all violence in 48 hours to salvage his peace plan.
He said in Tehran that he and his host agreed on the need to “find a peaceful solution to the crisis” and he voiced optimism that the situation on the ground would improve by Thursday morning.
Mr Salehi insisted that “change in Syria” should come under the leadership of Assad.
Earlier, US secretary of state Hillary Clinton vowed to pressure Russia over the crisis, saying its refusal to support action by the United Nations Security Council was keeping Assad in power.
Mrs Clinton said the US would again try to persuade Russia, a key Syrian ally, to support action that would at least allow humanitarian access, when foreign ministers of the G8 meet in Washington today.
She warned that the danger was rising of regional conflict and civil war flaring from the violence in Syria.
Russia’s “refusal to join us in some kind of constructive action is keeping Assad in power, well-armed, able to ignore the demands of his own people, the region and the world”, she said.