The miserable event took place following the UN observers visit to the Arbaeen neighborhood within the Hama city. The city of Hama is actively involved in revolting against President Bashar al-Assad. The UN observers are in Syria to witness the 11-day-old truce agreement.

"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 agreement, 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. (Reuters)