At least 13 people were killed Thursday in the latest government blitz on the besieged Syrian city of Homs, activists said, adding that the toll was likely to rise.

"Twelve of the victims died in shelling that began at dawn on the Baba Amr neighbourhood and one died in Khaldiyeh," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP.

"But we expect more victims because two families whose homes were shelled in Baba Amr remain under the rubble," said the head of the Britain-based group.

Abdel Rahman said that the government assault on the central city which began on Saturday has left more than 400 people dead.

Ali Hazuri, a doctor in Baba Amr reached by telephone from Beirut, said the shelling had eased overnight but resumed at daybreak on Thursday.

"The shells are raining down on us and regime forces are using heavy artillery," he said.

Omar Shaker, an activist in Baba Amr also reached by phone, added that residents of the neighbourhood were hiding on the ground floor of buildings as there were no underground shelters.

"When you venture outside, you can see craters every 10 meters (yards)," he said.