More than 30 people died in Latamneh, a suburb of the city of Hama, in government shelling, clandestine monitors told the BBC.
Activists' videos showed dead bodies being piled in flatbed vehicles and burials, while a dead toddler was being held aloft.
The British broadcaster said video reportedly from Homs, the site of heavy attacks from tanks and artillery, showed 13 victims of what was apparently a mass execution, with 13 dead bodies next to a wall stained with blood and marked with bullet holes.
U.N. Secretary-General Ban Ki-moon warned Friday through a spokesman the approaching Tuesday deadline for Syria to withdraw its military forces from cities and towns is "not an excuse for continued killing."
"The Syrian authorities remain fully accountable for grave violations of human rights and international humanitarian law. These must stop at once," the spokesman said.
In letters to the United Nations, the Syrian government has blamed "armed terrorist groups" for the violence, which the government said has worsened since the peace plan negotiated by U.N.-Arab League envoy Kofi Annan was accepted.
Annan has said if the truce succeeds, a U.N. monitoring mission of 200-250 observers could be sent to Syria.
The United Nations estimates more than 9,000 people have been killed in the yearlong uprising against Syrian President Bashar Assad. Another 1 million people need humanitarian aid inside the country and tens of thousands have been displaced.