The violence happened as a three-day ceasefire, brokered last week in rebel-held parts of Homs to allow the evacuation of civilians and humanitarian aid convoys to come in, was shattered when trucks with supplies came under fire in Homs on Saturday. The ceasefire expired Monday.
On Sunday, 600 people had been rescued as fighting broke out.
The British-based Syrian Observatory for Human Rights said half the victims in the attack in Maan were civilians, while the other half were villagers defending their homes.
Syrian state media described the attack as a "massacre" perpetrated by rebels fighting to topple President Bashar al-Assad.
In Geneva, representatives from the Syrian government are meeting Monday with members of the opposition, beginning the second round of negotiations aimed at ending the nearly three-year civil war, the CBC's Derek Stoffel reported from Jerusalem.
The weekend rescue of people took place as aid workers delivered much-needed supplies to Homs, where 2,500 people have been trapped with little food or medicine. Although the two sides had negotiated the weekend rescue, it took place amid sniper fire and exploding mortar shells, media reports said.
"In Syria these days, a short ceasefire is seen as a victory," Stoffel said. "There are few who hold any hope that these talks in Geneva will bring an end to three years of suffering, three years of bloody civil war."
KhaledErksoussi, of the Syrian Red Crescent, hopes the ceasefire can be extended, to deliver more aid and evacuate more people from Homs.
"We know about more people need and want to be evacuated," he said. "What we are trying to do now is push new, safe exit point."
Mediator seeks declaration
He told The Associated Press by phone from Damascus that there are 200 families, mostly Christians, who want to leave two rebel-held districts in central Homs. Those people were unable to reach the designated exit point from the city during the ceasefire.
In Geneva, Lakhdar Brahimi, the Algerian United Nations envoy and adviser who is acting as mediator at talks between Syria and rebels, asked the two sides to declare that they have the political will to resolve the issues on the table.
Brahimi told delegations he plans to address issues of stopping the violence and establishing transitional governing body in parallel, according to a document obtained by Reuters.
As well, he plans to keep meeting delegations separately for next two or three days, hoping for atmosphere at the talks to improve, Reuters said.