The Syrian Observatory for Human Rights reported that dozens of civilians were killed on Wednesday. The deaths were caused by shelling and gunfire from Syrian forces, and three entire families were also killed overnight in the central city by regime-backed thugs, known as Shabiha.

"We expect the death toll to rise ... given the fact that many victims remain under the rubble," Rami Abdel Rahman, head of the Britain-based group, told AFP.

Activists said the most intense shelling appeared to be in the neighbourhood of Baba Amr, where buildings in some areas were completely flattened and all power and communications were cut off.

"Since dawn the shelling has been extremely intense and they are using rockets and mortars," said Omar Shaker, an activist in the besieged city reached by satellite phone from Beirut.

"They have destroyed all infrastructure and bombed water tanks and electricity poles," he added. "The humanitarian situation is extremely dire and food is lacking.

"We are trying to set up a field hospital but we have no medical supplies."

He said at least 40 percent of buildings in Baba Amr had been hit.

Shaker said it was clear the intense shelling was aimed at paving the way for a ground assault.

"The shelling right now is from far away which means troops are not moving on the ground," he said. "When the shelling eases it means they are moving."

The Observatory said that apart from Baba Amr the shelling was also targeting the Khaldiyeh, Bayada, Karm el-Zeytoun and Wadi Iran neighbourhoods.

The three families killed were in the Assabil and Nazihin neighborhoods.

The Observatory has reported several hundred civilians killed since the onslaught was launched overnight Friday.

Activists say more than 6,000 people have died in Syria since the outbreak of a popular revolt last March against the regime of Bashar al-Assad.