The UK-based monitoring group said women and children were among the 106 people killed by forces who stormed Basatin al Hasawiya, an impoverished district on the edge of the town.
Reports could not be verified due to reporting restrictions in Syria.
Head of the Syrian Observatory for Human Right, Rami Abdelrahman, said: "The Observatory has the names of 14 members of one family, including three children, and information on other families who were killed, including one of 32 people.
"This needs to be investigated by the United Nations."
Abu Yazen, an opposition activist in Homs, said the rebel Free Syrian Army occasionally entered the district to attack a nearby military academy.