Twenty-one people, including four children and six women, were killed when a minibus exploded in the southern town of Noa early on Wednesday, the Syrian Observatory for Human Rights monitoring group said.
"Twenty-one people were killed in the Nawa area (of Deraa), among them four children and six women, in a blast that detonated as their vehicle went past Tal al-Jumaa," AFP quoted the Observatory as reporting.
Opposition activists told the Observatory the minibus drove over a mine planted by forces loyal to President Bashar Assad. There was no immediate comment from the government.
The explosion was reported in rebel-held territory but there are also army troops in the nearby base of Tel al-Jumaa, which is besieged.
Violence has continued despite pleas from regional Arab and Muslim organizations for a ceasefire to mark the Muslim festival of Eid al-Adha.
On Monday, the Observatory said a car bomb killed at least 20 people on Monday in the northern Syrian town of Darkoush, close to the Turkish border.
It said dozens of people were wounded by the explosion in the market of Darkoush, a small rebel-controlled town 2 km (1.5 miles) from the frontier, and some of them were taken into Turkey for treatment.
Crisis in Syria - full JPost.com coverage
The British-based group, which monitors violence in Syria through a network of activists and medical and military sources says more than 115,000 people have been killed in Syria's 2-1/2 year conflict, which grew out of protests that erupted in March 2011 against President Bashar Assad's rule.