Syria - More than 80 people were killed in Syria Thursday, most of them in a new regime blitz on the city of Homs, an attack US President Barack Obama decried as "outrageous bloodshed".

Shelling erupted at daybreak, killing more than 50 civilians in the besieged central city and burning several bodies beyond recognition, the Britain-based Syrian Observatory for Human Rights said.

Troops trying to crush opponents of President Bashar al-Assad have killed at least 400 people in a relentless six-day onslaught on Homs, opposition activists say.

"The shells are raining down on us and regime forces are using heavy artillery," said Ali Hazuri, a doctor in the Baba Amr district reached by telephone from Beirut.