Damascus: Syrian forces killed 57 civilians on Wednesday as they blitzed the city of Homs and a village in Idlib province, monitors said. The Red Cross sought a truce to deliver aid and the United Nations demanded unimpeded access for aid groups.

The escalation comes as Russia, a key ally of President Bashar al-Assad's regime, said it will boycott an international conference in Tunis this week aimed at seeking political change in Syria and China refused to commit.

It also comes ahead of the weekend referendum on a new constitution that could end nearly five decades of rule by Assad's Baath party.

At least 33 civilians were killed in an operation by Syrian forces in the village of Abdita in the northwestern province of Idlib that extended to the neighbouring villages of Ibleen and Balshon, the Syrian Observatory for Human Rights said

And despite a plea by activists on Tuesday to allow women and children to flee Homs' besieged Baba Amr neighbourhood, troop reinforcements were sent to the outskirts of the restive city, which residents say has been under assault for 18 straight days.

Activists fear they are preparing to storm its defiant neighbourhoods.

The Observatory said 16 people, including three children, died in "intensive shelling" that targeted Baba Amr, with the Khaldiyeh and Karm al-Zaytoun districts also blasted.

Eight more civilians, including a child, were reportedly killed by gunfire elsewhere in the country.

The International Committee of the Red Cross called for a daily truce of two hours in Syria so it can deliver vital aid to afflicted areas, after saying a day earlier it was in talks with both sides to halt the violence.