Security forces in the Syrian city of Hama dragged an old woman into the street, hit her and shot her dead with two of her children hours after a visit by UN observers, activists have claimed.
Residents of Hama quoted by Avaaz said the attacks were in "revenge" for large-scale protests during the UN visit.
The advance UN team is in Syria to monitor a fragile and seemingly crumbling ceasefire, based on a plan drawn up by international envoy Kofi Annan.
Around 10 observers are currently in Syria, ahead of the deployment of a peacekeeping force of around 300 that was agreed over the weekend by the UN Security Council.
But as widespread violence continues to be reported across the country many are losing faith that President Assad's government will ever fully comply with peace plan.
Some of the UN observation team may have still been in Hama when 50 homes were destroyed and 36 people killed in heavy shelling on Monday, Avaaz said.
Activist Mosaab Abo Jehad told Avaaz "The UN observer mission visited the neighbourhood yesterday and a large protest came out to welcome them despite the military and security snipers present on the building in front of the UN observers.
"As soon as the UN observer mission left the neighbourhood the security forces began threatening us in the neighbourhood that they would take 'revenge'.
"At the start we thought the army was going to raid homes and search them as usual, but the army began shelling us really heavily.
"Some families began fleeing from their homes but many others refused to leave. The security men were storming homes, if they found people inside they were taken out and shot immediately."
Video footage posted online claimed to show the dead body of an elderly woman who was shot in the street with her children. The extremely graphic footage was viewable on YouTube as of press time, having been made available by citizen journalists in Hama.
Other video footage showed security forces threatening families in the affected neighbourhoods.
It was not possible to independently verify the activists' claims.
A report on a state media website said that security forces had searched the "terrorists'" homes "in cooperation with the families.
"The competent authorities, in cooperation with the families, pursued the terrorists and clashed with them, arresting a number of them, seizing their weapons and restoring a big number of the stolen cars," said a report on Sana.sy.
The government also claimed that "armed terrorist groups" assassinated a doctor in Daraa, and two military personnel in Hama.
The UN says many more than 9,000 people have been killed in Syria since anti-government protests began in March 2011.