Syrian forces shelled a district of the central city of Hama on Monday and opened fire on residents, killing at least 20 people, activists said.
The incident oocurred in the Arbaeen neighbourhood a day after United Nations observers, monitoring an 11-day-old ceasefire agreement, visited Hama, a hotbed of a popular revolt against President Bashar al-Assad.
"It began in the morning with tanks and artillery, there were houses burning," a local activist called Mousab told Reuters by telephone.
"The military forces entered and shot people in the street".
At least 20 people were killed, 60 were wounded and more could be stuck under collapsed buildings, he said.
There was no immediate comment from Syrian authorities who say they are committed to international mediator Kofi Annan's April 12 ceasefire agremeent, but reserve the right to respond to what they say are continued attacks by "terrorist groups".
Assad's father, Hafez al-Assad, crushed an armed Islamist uprising in Hama 30 years ago, killing many thousands of people and flattening parts of Hama's old city.
Rami Abo Adnan, an activist living in a district of Hama next to Arbaeen, said he could see a thick pillar of smoke rising from Arbaeen where the shells had landed.
"There was shelling earlier, then the security forces came in and started shooting and burning houses," he said.